Multi-cell power conversion method and multi-cell power converter

ABSTRACT

A method includes converting power by a power converter comprising a plurality of converter cells and at least one filter cell, receiving a cell input power at a cell input and providing a cell output power at a cell output of at least one of the plurality of converter cells, and operating the filter cell in one of an input power mod, in which the filter cell receives an input power, and an output power mode, in which the filter cell provides an output power.

TECHNICAL FIELD

This disclosure in general relates to power conversion methods, and topower converters.

BACKGROUND

Power conversion is an important issue in many different electronicapplications. An important aspect in almost every kind of powerconversion is to efficiently convert the power, that is, to keep lossesthat may occur in connection with the power conversion as low aspossible.

SUMMARY

According to one embodiment, a method includes converting power by apower converter comprising a plurality of converter cells and at leastone filter cell, receiving a cell input power at a cell input andproviding a cell output power at a cell output of at least one of theplurality of converter cells, and operating the filter cell in one of aninput power mode, in which the filter cell receives an input power, andan output power mode in, in which the filter cell provides an outputpower.

According to one embodiment, a power converter circuit includes aplurality of converter cells, wherein at least one of the plurality ofconverter cells is configured to receive a cell input power at a cellinput and provide a cell output power at a cell output, and at least onefilter cell configured to operate in one of an input power mode, and anoutput power mode in. In the input power mode, the filter cell receivesan input power, and in the output power mode, the filter cell providesan output power.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples are explained below with reference to the drawings. Thedrawings serve to illustrate certain principles, so that only aspectsnecessary for understanding these principles are illustrated. Thedrawings are not to scale. In the drawings the same reference charactersdenote like features.

FIG. 1 illustrates a power converter circuit with two power converters.

FIGS. 2A-2C show timing diagrams which illustrate some different typesof power conversion methods.

FIGS. 3A-3C show timing diagrams which illustrate some different typesof power conversion methods.

FIG. 4 shows one embodiment of a power converter circuit having an ISOP(Input Serial, Output Parallel) topology.

FIG. 5 shows one embodiment of a power converter circuit having an ISOS(Input Serial, Output Serial) topology.

FIG. 6 shows one embodiment of a power converter circuit having an IPOS(Input Parallel, Output Serial) topology.

FIG. 7 shows one embodiment of a power converter circuit having an IPOP(Input Parallel, Output Parallel) topology.

FIG. 8 shows two converter cells of a power converter circuit from whichone has an isolating topology and one has a non-isolating topology.

FIG. 9 shows two converter cells of a power converter circuit from whichone has an isolating topology and one has a non-isolating topology.

FIG. 10 shows two converter cells of a power converter circuit, bothhaving a non-isolating topology.

FIG. 11 shows one embodiment of a rectifier circuit.

FIG. 12 shows one embodiment of a multi-cell power converter with an IS(Input Serial) topology.

FIG. 13 shows one embodiment of a main controller in the a multi-cellpower converter shown in FIG. 12.

FIG. 14 shows one embodiment of the controller shown in FIG. 13 ingreater detail.

FIG. 15 schematically illustrates one embodiment of an input voltage ofthe multi-cell power converter shown in FIG. 12, and of a correspondingmodulation index.

FIG. 16 shows one embodiment of a cell controller of one converter cellin the multi-cell power converter shown in FIG. 12.

FIG. 17 shows timing diagrams which illustrate one way of operation ofthe controller shown in FIG. 16.

FIG. 18 shows one embodiment of a PWM controller in the cell controllershown in FIG. 17.

FIGS. 19A-19B show timing diagrams which illustrates one way ofoperation of the multi-cell power converter shown in FIG. 12 atdifferent modulation indices.

FIG. 20 schematically illustrates one embodiment of an input voltagewaveform of the multi-cell power converter shown in FIG. 12, and of acorresponding total cell input voltage.

FIG. 21 illustrates how cell controllers in individual converter cellsof the multi-cell power converter shown in FIG. 12 may be synchronized.

FIG. 22 shows a modification of the PWM controller shown in FIG. 18.

FIG. 23 shows timing diagrams which illustrate another way of operationof the multi-cell power converter shown in FIG. 12.

FIG. 24 shows a converter cell of a multi-cell power converter accordingto one embodiment.

FIGS. 25A-25B show timing diagrams which illustrate one way of operationof the converter cell shown in FIG. 24.

FIGS. 26A-26B show two embodiments of a cell controller in the convertercell shown in FIGS. 25A-25B.

FIG. 27 schematically illustrates one embodiment of an input voltagewaveform of a multi-cell power converter shown in FIG. 12 whenimplemented with a converter cell shown in FIG. 24, and of acorresponding total cell input voltage.

FIG. 28 shows a modification of the main controller shown in FIG. 14.

FIG. 29 shows one embodiment of a multi-cell power converter with an IP(Input Parallel) topology.

FIG. 30 shows one embodiment of a controller in one of the convertercells shown in FIG. 29.

FIG. 31 shows one embodiment of a multi-cell power converter with an OP(Output Parallel) topology.

FIGS. 32A-32B shows two embodiments of a converter cell which may beused in the multi-cell power converter shown in FIG. 31.

FIG. 33 shows one embodiment of a main controller in the multi-cellpower converter shown in FIG. 31.

FIG. 34 shows one embodiment of a multi-cell power converter with an OS(Output Serial) topology.

FIG. 35 shows one embodiment of a main controller of the multi-cellpower converter shown in FIG. 34.

FIG. 36 shows one embodiment of the main controller shown in FIG. 35 ingreater detail.

FIG. 37 shows one embodiment of a multi-cell power converter with an OP(Output Parallel) topology.

FIG. 38 shows one embodiment of a multi-cell power converter with an IP(Input Parallel) topology.

FIG. 39 shows one embodiment of a main controller in the multi-cellpower converter shown in FIG. 38.

FIG. 40 schematically illustrates the efficiency of a converter cellbased on the power level of the converted power.

FIGS. 41A-41B show timing diagrams which illustrate activating anddeactivating converter cells (phase shedding) in a multi-cell powerconverter with OP topology.

FIG. 42 schematically illustrates how a number of inactive convertercells may be set in a multi-cell power converter with OP topologydependent on an output power.

FIG. 43 illustrates one embodiment of operating a multi-cell converterwith OP topology.

FIG. 44 shows one embodiment of a main controller in a multi-cell powerconverter having a phase shedding functionality.

FIGS. 45A-45B show timing diagrams which illustrate activating anddeactivating converter cells (phase shedding) in a multi-cell powerconverter with IP topology.

FIG. 46 schematically illustrates how a number of inactive convertercells may be set in a multi-cell power converter with IP topologydependent on an output power.

FIG. 47 illustrates one embodiment of operating a multi-cell converterwith OP topology.

FIG. 48 shows one embodiment of a main controller in a multi-cell powerconverter having a phase shedding functionality.

FIG. 49 shows one embodiment of a main controller in a multi-cell powerconverter having a phase shedding functionality.

FIG. 50 shows timing diagrams which illustrate one way of operation ofan IS or OS multi-cell converter in an intermittent operation mode.

FIG. 51 shows one embodiment of a main controller in an IS multi-cellconverter having an intermittent operation functionality.

FIG. 52 shows one embodiment of a main controller in an IS multi-cellconverter having an intermittent operation functionality.

FIG. 53 shows timing diagrams which illustrate one way of operation ofan IP or OP multi-cell converter in an intermittent operation mode.

FIG. 54 shows timing diagrams which illustrate one way of operation ofan IP or OP multi-cell converter in an intermittent operation mode.

FIG. 55 shows one section of a power converter circuit which includes anoutput capacitor.

FIG. 56 illustrates one embodiment of operating a multi-cell converterwith OP topology in an intermittent operation mode.

FIG. 57 illustrates one embodiment of operating a multi-cell converterwith IP topology in an intermittent operation mode.

FIG. 58 shows one embodiment of a main controller in a multi-cellconverter with OP topology.

FIG. 59 shows one embodiment of a main controller in a multi-cellconverter with IP topology.

FIG. 60 shows one embodiment of a multi-cell converter which includes afilter cell.

FIG. 61 shows one embodiment of the filter cell shown in FIG. 60.

FIG. 62 shows one embodiment of a main controller in the multi-cellconverter shown in FIG. 60.

FIG. 63 shows timing diagrams which illustrate one way of operation ofthe multi-cell converter shown in FIG. 60.

FIG. 64 shows one embodiment of operating the multi-cell converter shownin FIG. 60.

FIG. 65 shows timing diagrams which illustrate one way of operation ofthe multi-cell converter shown in FIG. 60.

FIG. 66 shows one embodiment of a multi-cell converter which includes afilter cell.

FIG. 67 shows one embodiment of the filter cell shown in FIG. 66.

FIG. 68 shows one embodiment of a main controller in the multi-cellconverter shown in FIG. 66.

FIG. 69 shows one embodiment of operating the multi-cell converter shownin FIG. 66.

FIG. 70 shows two converter cells of a multi-cell converter and a switchcircuit which connects the cell inputs either in series or in parallel.

FIG. 71 shows timing diagrams which illustrate one way of operation ofthe converter cells shown in FIG. 70.

FIG. 72 shows one embodiment of a main controller in a multi-cellconverter with two rearrangeable converter cells as shown in FIG. 70.

FIG. 73 shows two converter cells of a multi-cell converter and a switchcircuit which connects the cell inputs either in series or in parallel.

FIG. 74 shows timing diagrams which illustrate one way of operation ofthe converter cells shown in FIG. 73.

FIG. 75 shows one embodiment of a main controller in a multi-cellconverter with two rearrangeable converter cells as shown in FIG. 73.

FIGS. 76A-76B illustrate an unequal distribution of power shares andcurrent shares, respectively, in a multi-cell converter.

FIG. 77 shows one embodiment of a main controller configured to controla distribution of power and current shares, respectively, in amulti-cell converter with an IP topology.

FIG. 78 shows one embodiment of a main controller configured to controla distribution of power and current shares, respectively, in amulti-cell converter with an OP topology.

FIGS. 79A-79B show timing diagrams which illustrate operation of amulti-cell converter with an IS or OS topology such that DC linkvoltages have different voltage levels.

FIG. 80 shows one embodiment of a main controller configured to operatea multi-cell converter as shown in FIGS. 79A-79B.

FIG. 81 shows one embodiment of a half-bridge in a converter cell ofmulti-cell converter.

FIG. 82 shows timing diagrams which illustrate a PWM operation of thehalf-bridge shown in FIG. 81.

FIG. 83 illustrates losses that occur in the half-bridge shown in FIG.81 at different duty cycles of a PWM operation for several differentdesigns of the half-bridge.

FIG. 84 shows one embodiment of a method for optimizing operation of amulti-cell power converter by differently operating the individualconverter cells.

FIG. 85 shows one embodiment of a main controller in an IS multi-cellconverter with an optimization functionality as illustrated in FIG. 84.

FIG. 86 shows one embodiment of a main controller in an OS multi-cellconverter with an optimization functionality as illustrated in FIG. 84.

FIG. 87 shows one embodiment of a bridge circuit in a multi-cellconverter.

FIG. 88 shows one embodiment of a power converter circuit which includesa multi-cell converter and a single cell converter.

FIG. 89 shows one embodiment of a power converter circuit which includesa multi-cell converter which receives a plurality of DC voltages fromdifferent power sources.

FIG. 90 shows one embodiment of a power converter circuit which includesa multi-cell converter and a plurality of single cell converters coupledto the multi-cell converter.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings. The drawings form a part of the description andby way of illustration show specific embodiments in which the inventionmay be practiced. It is to be understood that the features of thevarious embodiments described herein may be combined with each other,unless specifically noted otherwise.

In the following, several embodiments of power conversion methods andpower converter circuits are explained with reference to drawings. Thesepower converter circuits include at least one power converter withseveral power converter cells. A power converter including several powerconverter cells will be referred to as multi-cell power converter ormulti cell converter in the following. A power conversion method whichuses at least one multi-cell converter will be referred to as multi-cellpower conversion method.

FIG. 1 shows one embodiment of a power converter circuit which isconfigured to convert an input power P_(IN) received at an input IN1,IN2 into an output power P_(OUT) provided at an output OUT1, OUT2. Theinput power P_(IN) is defined as the product of an input current I_(IN)received at the input IN1, IN2, and an input voltage V_(IN) between afirst input node IN1 and a second input node IN2 of the input, so thatP_(IN)=V_(IN)·I_(IN). The output power P_(OUT) is defined as the productof an output current I_(OUT) provided at the output OUT1, OUT2, and anoutput voltage V_(OUT) between a first output node OUT1 and a secondoutput node OUT2 of the output, so that P_(OUT)=V_(OUT)·I_(OUT). A loadZ (illustrated in dashed lines in FIG. 1) may receive the output powerP_(OUT) provided by the second power converter 20.

The power converter circuit includes a first converter 10 configured toreceive the input power at the input IN1, IN2, and a second powerconverter 20 configured to provide the output power at the output OUT1,OUT2. At least one of the first power converter 10 and the second powerconverter includes a plurality of power converter cells, which willbriefly be referred to as converter cells in the following. In theembodiment shown in FIG. 1, the first power converter 10 includes aplurality of converter cells 1 ₁-1 _(N1), and the second power converter20 includes a plurality of converter cells 2 ₁-2 _(N3). These convertercells are only schematically illustrated in FIG. 1. The first powerconverter 10 and the second power converter 20 are linked by a pluralityof capacitors 11 ₁-11 _(N2). These capacitors 11 ₁-11 _(N2) will also bereferred to as DC link capacitors in the following. Capacitances of theindividual DC link capacitors are dependent on several aspects, such asa waveform of the input voltage and/or the output voltage, or a powerrating of the power converter circuit, to name only a few. According toone embodiment, the capacitances of the DC link capacitors are selectedfrom a range of between several microfarads (μF), such as 2 uF, andseveral millifarads (mF), such as 9 mF.

Referring to FIG. 1, the first power converter 10 may include a firstnumber N1 of converter cells 1 ₁-1 _(N1), a second number N2 ofcapacitors 11 ₁-11 _(N2) may link the first power converter 10 and thesecond power converter 20, and the second power converter 20 may includea third number N3 of converter cells 2 ₁-2 _(N2). According to oneembodiment, the first number N1, the second number N2, and the thirdnumber N3 are equal, so that N1=N2=N3=N.

Dependent on how the first power converter 10 and the second powerconverter 20 are implemented, different types of power conversionmethods may be performed by the power converter circuit. Some of thosedifferent types of power conversion methods are explained with referenceto FIGS. 2A3C below. Each of these FIGS. 2A-3C schematically illustratestiming diagrams of the input voltage V_(IN) and the output voltageV_(OUT).

Referring to FIG. 2A, the input voltage V_(IN) may be a rectified sinevoltage and the output voltage V_(OUT) may be a direct voltage with avoltage level lower than a peak voltage of the input voltage V_(IN).Referring to FIG. 2B, the input voltage V_(IN) may be a sine voltage andthe output voltage V_(OUT) may be a direct voltage with a voltage levellower than the amplitude of the input voltage V_(IN). A rectified sinevoltage as shown in FIG. 2A may be obtained by rectifying a sine voltageas shown in FIG. 2B. According to one embodiment, the sine voltage is agrid voltage with 110V_(RMS) or 220V_(RMS) and a frequency of 50 Hz or60 Hz. The type of power conversion shown in FIG. 2B may be used in avariety of different applications in which a load Z is to be suppliedwith DC power from a power grid. Examples of those applications includetelecommunication switches, computers, or the like. Referring to FIG. 2Ceach of the input voltage V_(IN) and the output voltage V_(OUT) may be adirect voltage, wherein a voltage level of the input voltage is V_(IN)is higher than a voltage level of the output voltage V_(OUT).

Referring to FIG. 3A, the output voltage V_(OUT) may be a rectified sinevoltage and the input voltage V_(IN) may be a direct voltage with avoltage level lower than a peak voltage of the output voltage V_(OUT).Referring to FIG. 3B, the output voltage V_(OUT) may be a sine voltageand the input voltage V_(IN) may be a direct voltage with a voltagelevel lower than the amplitude of the output voltage V_(OUT). Accordingto one embodiment, the sine voltage shown in FIG. 3B is a grid voltagewith 110V_(RMS) or 220V_(RMS) and a frequency of 50 Hz or 60 Hz. Thetype of power conversion shown in FIG. 3B may be used in applications inwhich power is to be supplied to a power grid from a DC power source,such as, for example a photovoltaic panel, a battery, or the like.Referring to FIG. 3C each of the input voltage V_(IN) and the outputvoltage V_(OUT) may be a direct voltage, wherein a voltage level of theinput voltage V_(IN) is lower than a voltage level of the output voltageV_(OUT).

In the embodiments shown in FIGS. 2A-2C, the voltage level of the outputvoltage V_(OUT) is lower than the voltage level or the effective voltagelevel of the input voltage V_(IN). In the following, a power convertercircuit configured to perform one of these types of power conversionwill be referred to as a power converter circuit having a step-downcharacteristic. In the embodiments shown in FIGS. 3A-3C, the voltagelevel of the input voltage V_(IN) is lower than the voltage level or theeffective voltage level of the output voltage. In the following, a powerconverter circuit configured to perform one of these types of powerconversion will be referred to as a power converter circuit having astep-up characteristic.

Four different power converter circuits of the type shown in FIG. 1 areexplained with reference to FIGS. 4-7 below. In each of theseembodiments, each of the first power converter 10 and the second 20power converter 20 includes a plurality of converter cells. Further, ineach of these embodiments, N1=N2=N3=N so that each of the plurality ofconverter cells in the first power converter 10 is linked to one of theplurality of converter cells in the second power converter 20 by one DClink capacitor. However, this is only example. Other examples where onlyone of the first and second power converters 10, 20 includes a pluralityof converter cells, or where at least two of N1, N2, and N3 aredifferent are explained further below. The power converter circuitsshown in FIGS. 4-7 are different in how the converter cells of the firstpower converter 10 are connected to the input IN1, IN2, and in how theconverter cells of the second power converter 20 are connected to theoutput OUT1, OUT2.

FIG. 4 shows one embodiment of a power converter circuit with an ISOP(Input Serial, Output Parallel) topology. In this power convertercircuit the converter cells 1 ₁-1 _(N1) of the first power converter 10are connected in series at the input IN1, IN2, and the converter cells 2₁-2 _(N3) of the second power converter 20 are connected in parallel atthe output OUT1, OUT2. This is explained below.

The converter cells 1 ₁-1 _(N1) of the first power converter 10 willalso be referred to as first converter cells in the following. Each ofthese first converter cells 1 ₁-1 _(N1) includes a cell input and a celloutput. The cell output of each converter cell 1 ₁-1 _(N1) is connectedto one of the plurality of DC link capacitors 11 ₁-11 _(N1), namely tothe DC link capacitor associated with the respective first convertercell 1 ₁-1 _(N1). The cell inputs of the first converter cells 1 ₁-1_(N1) are connected in series at the input IN1, IN2 of the powerconverter circuit. That is, a first cell input node of one of theplurality of first converter cells (the converter cell 1 ₁ in theembodiment shown in FIG. 4) is connected to the first input node IN1. Asecond cell input node of another one of the plurality of firstconverter cells (the first converter cell 1 _(N1) in the embodimentshown in FIG. 1) is connected to the second input node IN2 of the powerconverter circuit. The other first converter cells (the converter cells1 ₂, 1 ₃ shown in FIG. 1) each have the first cell input node connectedto the second cell input node of another first converter cell, and havetheir second cell input node connected to the first cell input node ofanother first converter cell. In other words, the cell inputs of theindividual first converter cells 1 ₁-1 _(N1) form a cascade between theinput nodes IN1, IN2 of the power converter circuit.

The converter cells 2 ₁-2 _(N3) of the second power converter 20 willalso be referred to as second converter cells in the following. Each ofthese second converter cells 2 ₁-2 _(N3) includes a cell input and acell output. The cell input of each converter cell 2 ₁-2 _(N3) isconnected to one of the plurality of DC link capacitors. The celloutputs of the second converter cells 2 ₁-2 _(N3) are connected inparallel at the output OUT1, OUT2 of the power converter circuit. Thatis, each of the second converter 2 ₁-2 _(N3) cells has a first celloutput node connected to a first output node OUT1 of the power convertercircuit, and each of the second converter cells 2 ₁-2 _(N3) has a secondcell output node connected to a second output node OUT2 of the powerconverter circuit.

In the power converter circuit with the ISOP topology shown in FIG. 4,each of the series connected first converter cells 1 ₁-1 _(N1) receivesa share or portion of the input voltage V_(IN) as a cell input voltageV1 ₁-V1 _(N1). That is, a sum of the cell input voltages V1 ₁-V1 _(N1)corresponds to the input voltage V_(IN),

V _(IN)=Σ_(i=1) ^(N1) V1_(i)  (1).

A cell input current of each first converter cell 1 ₁-1 _(N1) equals theinput current I_(IN). Further, each of the parallel connected secondconverter cells 2 ₁-2 _(N3) provides a cell output current I2 ₁-I2 _(N3)which is a share or portion of the output current I_(OUT). That is, asum of the cell output currents I2 ₁-I2 _(N3) corresponds to the outputcurrent I_(OUT),

I _(OUT)=Σ_(i=1) ^(N3) I2_(i)  (2).

A cell output voltage of each of the second converter cells correspondsto the output voltage V_(OUT) of the power converter circuit.

FIG. 5 shows one embodiment of a power converter circuit with an ISOS(Input Serial, Output Serial) topology. Like in the power convertercircuit shown in FIG. 4, the first converter cells 1 ₁-1 _(N1) areconnected in series at the input IN1, IN2. The power converter circuitshown in FIG. 5 is different from the power converter circuit shown inFIG. 4 in that the second converter cells 2 ₁-2 _(N3) are connected inseries at the output OUT1, OUT2. This is explained below.

Referring to FIG. 5, a first cell output node of one of the plurality ofsecond converter cells (the converter cell 2 ₁ in the embodiment shownin FIG. 5) is connected to the first output node OUT1. A second celloutput node of another one of the plurality of second converter cells(the second converter cell 2 _(N3) in the embodiment shown in FIG. 5) isconnected to the second output node OUT2 of the power converter circuit.The other second converter cells (the converter cells 2 ₂, 2 ₃ shown inFIG. 5) each have the first cell output node connected to the secondcell output node of another second converter cell, and have their secondcell output node connected to the first cell output node of anothersecond converter cell. In other words, the cell outputs of theindividual second converter cells 2 ₁-2 _(N3) form a cascade between theoutput nodes OUT1, OUT2 of the power converter circuit. In thisembodiment, a cell output voltage V3 ₁-V3 _(N3) of each of the pluralityof second converter cells 2 ₁-2 _(N3) is a share of the output voltageV_(OUT) of the power converter circuit. That is,

V _(OUT)=Σ_(i=1) ^(N3) V3_(i)  (3).

A cell output current of each of the plurality of second converter cells2 ₁-2 _(N3) equals the output current of the power converter circuit.

In the power converter circuit shown in FIG. 5, like in the powerconverter circuit shown in FIG. 4, the cell output of each of the firstconverter cells 1 ₁-1 _(N1) is connected to one of the plurality of DClink capacitors 11 ₁-11 _(N2), and the cell input of each of the secondconverter cells 2 ₁-2 _(N3) is connected to one of the DC linkcapacitors 11 ₁-11 _(N2), wherein each of the DC link capacitors 11 ₁-11_(N2) has only one first converter cell and only one second convertercell connected thereto.

FIG. 6 shows one embodiment of a power converter circuit with an IPOS(Input Parallel, Output Serial) topology. Like in the power convertercircuit shown in FIG. 5, the second converter cells 2 ₁-2 _(N3) areconnected in series at the output OUT1, OUT2. The power convertercircuit shown in FIG. 6 is different from the power converter circuitshown in FIG. 5 in that the first converter cells 1 ₁-1 _(N1) areconnected in parallel at the input IN1, IN2. That is, each of the firstconverter cells 1 ₁-1 _(N3) has its first cell input node connected tothe first input node IN1 of the power converter circuit, and has itssecond cell input node connected to the second input node IN2 of thepower converter circuit. Thus, each of the first converter cells 1 ₁-1_(N3) receives the input voltage V_(IN) as a cell input voltage, and acell input current I0 ₁-I0 _(N1) of each of the first converter cells 1₁-1 _(N1) is a share or portion of the input current I_(OUT), so that

I _(IN)=Σ_(i=1) ^(N1) I0_(i)  (4).

FIG. 7 shows one embodiment of a power converter circuit with an IPOP(Input Parallel, Output Parallel) topology. In this power convertercircuit, the first converter cells 1 ₁-1 _(N1) are connected in parallelat the input IN1, IN2, and the second converter cells 2 ₁-2 _(N3) areconnected in parallel at the output. Concerning the parallel connectionof the first converter cells 1 ₁-1 _(N1) reference is made to FIG. 6 andthe corresponding description, and concerning the parallel connection ofthe second converter cells 2 ₁-2 _(N3) reference is made to FIG. 4 andthe corresponding description.

Each of the first converter cells 1 ₁-1 _(N1) and the second convertercells 2 ₁-2 _(N3) is implemented with a power converter topology and isconfigured to receive a cell input power at the cell input and provide acell output power at the cell output. Each of the first converter cells1 ₁-1 _(N1) receives its cell input power from the input IN1, IN2. Thecell output power of each of the first converter cells 1 ₁-1 _(N1) isthe power the respective first converter cells provides to the DC linkcapacitor connected to the cell output and to the second converter cellconnected to the cell output, respectively. The cell input power of eachof the plurality of second converter cells 2 ₁-2 _(N3) is the power, therespective second converter cell receives from the DC link capacitor itis connected thereto, and from the first converter it is connectedthereto, respectively. Each of the second converter cells provides itscell output power to the output OUT1, OUT1. The DC link capacitors 11₁-11 _(N2) are capable of storing energy so that the power level of thecell output power of one of the first converter cells 1 ₁-1 _(N1) andthe power level of the cell input power of the associated secondconverter cell can be different. In the following, the word “associated”is used to describe the relationship between one first converter cells,the DC link capacitor connected to this first converter cell, and thesecond converter cells connected to this first converter cell and thisDC link capacitor.

The type of converter topology implemented in the first converter cells1 ₁-1 _(N1) and the second converter cells 2 ₁-2 _(N3) is, for example,dependent on the type of power conversion the power converter circuitperforms. In general, the converter cells 1 ₁-1 _(N1), 2 ₁-2 _(N3) canbe implemented with an isolating power converter topology, or with anon-isolating power converter topology. In the first case, therespective converter cell includes a transformer which galvanicallyisolates the cell input and the cell output. In the second case, thecell input and the cell output of the converter cell are notgalvanically isolated. This is explained with reference to FIGS. 8-10below. Each of these Figures shows a first converter cell 1 _(i), the DClink capacitor 11 _(i) of the first converter cell 1 _(i) and a secondconverter cell 2 _(i) connected to the first converter cell 1 _(i). Thefirst converter cell 1 _(i) and the second converter cell 2 _(i)represent any pair of a first converter cell 1 ₁-1 _(N1) and a secondconverter cell 2 ₁-2 _(N3) connected to the first converter cell 1 ₁-1_(N1) in any of the power converter circuits explained herein before.

In the embodiment shown in FIG. 8, the first converter cell 1 _(i) isimplemented with an isolating converter topology. This is schematicallyillustrated by the transformer symbol in the circuit block representingthe first converter cell 1 _(i). The second converter cell 2 _(i) isimplemented with a non-isolating converter topology. In a powerconverter circuit in which the first converter cells 1 ₁-1 _(N1) areimplemented with an isolating converter topology, and the secondconverter cells 2 ₁-2 _(N3) are implemented with a non-isolatingconverter topology, as shown in FIG. 8, the first converter cells 1 ₁-1_(N1) provide for a galvanic isolation between the input IN1, IN2 andthe output OUT1, OUT2 of the power converter circuit.

In the embodiment shown in FIG. 9, the first converter cell 1 _(i) isimplemented with a non-isolating converter topology. This isschematically illustrated by the transformer symbol in the circuit blockrepresenting the second converter cell 2 _(i). The first converter cell1 _(i) is implemented with a non-isolating converter topology. In apower converter circuit in which the first converter cells 1 ₁-1 _(N1)are implemented with a non-isolating converter topology, and the secondconverter cells 2 ₁-2 _(N3) are implemented with an isolating convertertopology, as shown in FIG. 9, the second converter cells 2 ₁-2 _(N3)provide for a galvanic isolation between the input IN1, IN2 and theoutput OUT1, OUT2.

In the embodiment shown in FIG. 10, none of the first power convertercircuit 1 _(i) and the second power converter circuit 2 _(i) isimplemented with an isolating converter topology. According to anotherembodiment (not shown), both of the first power converter circuit 1 _(i)and the second power converter circuit 2 _(i) is implemented with anisolating converter topology.

In the following, different embodiments of the first power converter 10and ways of operation of these embodiments are explained. In thefollowing, a first multi-cell power converter 10 with series connectedconverter cells 1 ₁-1 _(N1) will be referred to as IS (Input Serial)converter, or power converter with an IS topology. Equivalently, a firstmulti-cell power converter 10 with parallel connected converter cells 1₁-1 _(N1) will be referred to as IP (Input Parallel) converter, or powerconverter with an IP topology. A second multi-cell power converter withseries connected converter cells 2 ₁-2 _(N3) will be referred to as OS(Output Serial) converter, or power converter with an OS topology.Equivalently, a second multi-cell power converter 20 with parallelconnected converter cells 1 ₁-1 _(N1) will be referred to as OP (OutputParallel) converter, or power converter with an OP topology. In contextwith one of the first and second power converters 10, 20, “seriesconnected converter cells” are converter cells which either have theircell inputs connected in series (in the first converter 10), or havetheir cell outputs connected in series (in the second converter 20), and“parallel connected converter cells” are converter cells which eitherhave their cell inputs connected in parallel (in the first converter10), or have their cell outputs connected in parallel (in the secondconverter 20).

At first, an embodiment of a first power converter 10 with an IStopology is explained which is configured to receive as an input voltageV_(IN) a rectified sine voltage, as shown in FIG. 2A, and to provide aplurality of DC link voltages V2 ₁-V2 _(N2) at the individual DC linkcapacitors 11 ₁-11 _(N2) (wherein N1=N2, in this embodiment). Referringto FIG. 11, such input voltage V_(IN) with a rectified sinusoidalwaveform may be obtained from a sinusoidal grid voltage V_(GRID) by abridge rectifier 100 with four rectifier elements 101-104. Theserectifier elements may be diodes, as shown in FIG. 11. However otherrectifier elements such as switches operated as synchronous rectifierelements may be used as well. These rectifier elements 101-104 areconnected in a bridgeconfiguration and receive the grid voltage V_(GRID)as an input voltage and provide the rectified sine voltage as an outputvoltage. This output voltage of the rectifier circuit 100 is the inputvoltage V_(IN) of the power converter circuit, from which only the inputIN1, IN2 is shown in FIG. 11.

The grid voltage V_(GRID) may be a 110 V_(RMS) or a 230 V_(RMS) sinevoltage. In the first case, a peak voltage of the rectified inputvoltage V_(IN) is about 160V and in the second case, the peak voltage isabout 320V. According to another embodiment, the grid voltage is amedium voltage with a peak voltage of up to several kilovolts (kV).

According to one embodiment, The first power converter 10 with theplurality of first converter cells 1 ₁-1 _(N1) is configured to generatethe DC link voltages V2 ₁-V2 _(N2) from the input voltage V_(IN) suchthat a voltage level of an overall DC link voltage (total DC linkvoltage) V2 ₁-V2 _(N) is higher than the voltage level of the peakvoltage of the input voltage V_(IN). The total DC link voltage V2 _(TOT)equals the sum of the individual DC link voltages V2 ₁-V2 _(N), that is,

V2_(TOT)=Σ_(i=1) ^(N2) V2_(i)  (5).

According to one embodiment, the total DC link voltage V2 _(TOT) isbetween 1.1 times and 1.3 times the peak voltage. For example, in caseof an input voltage V_(IN) obtained from a 220V_(RMS) sine voltage, thetotal DC link voltage V2 _(TOT) is about 400V.

FIG. 12 shows one embodiment of a first power converter 10 having an IStopology and configured to generate the total DC link voltage V2 _(TOT)with a higher voltage level than the peak voltage level of the inputvoltage V_(IN). In this embodiment, the individual first converterstages 1 ₁-1 _(N1) are each implemented with a boost converter topology,which is one type of a non-isolating converter topology. In FIG. 12,only one of the first converter cells 1 ₁-1 _(N1) namely the firstconverter cell 1 ₁ is shown in detail. The other first converter cells 1₂-1 _(N1) are implemented with the same topology. Thus, the explanationprovided in context with the first converter cell 1 ₁ equivalentlyapplies to the other first converter cells 1 ₂-1 _(N1).

Referring to FIG. 12, the first converter cells 1 includes a half-bridge12 with a low-side switch 12 _(L) and a high-side switch 12 _(H). Thehigh-side switch 12 _(H) is optional and may be replaced with arectifier element such as, for example, a diode. Referring to FIG. 12,the high-side switch may be implemented with an electronic switch and aparallel rectifier element. The electronic switch is operated as asynchronous rectifier which switches on each time the parallel rectifierelement is conducting. Thus, the high-side switch 12 _(H) operates likean active rectifier element. However, losses occurring in the high-sideswitch 12 _(H) when the switch is on are lower than losses that occur ina comparable passive rectifier element, such as a diode. The low sideswitch 12 _(L) may also be implemented with an electronic switch and aparallel rectifier element. However, the rectifier element is optionalin this embodiment. The high-side switch 12 _(H) and the low-side switch12 _(L) can be implemented as electronic switches. Examples of thoseswitches include, but are not restricted to, MOSFETs (Metal OxideSemiconductor Field-Effect Transistors), IGBTs (Insulated Gate BipolarTransistors), JFETs (Junction Field-Effect Transistors), BJTs (BipolarJunction Transistor), HEMTs (High Electron Mobility Transistor),GaNHEMTs, or the like. Some types of these electronic switches, such asMOSFETs, include an integrated diode (body diode) which may be used asthe rectifier element shown in FIG. 12.

Referring to FIG. 12, the low-side switch 12 _(L) is connected betweencell input nodes of the first converter cell 1 ₁. Thus the low-sideswitch 12 _(L) of the first converter cell 1 ₁ and correspondinglow-side switches (not shown) in the other first converter cells 1 ₂-1_(N1) form a series circuit which is connected between the input nodesof the input IN1, IN2. The high-side switch 12 _(H) and the DC linkcapacitor 11 ₁ of the first converter cell 1 ₁ form a series circuit,wherein this series circuit is connected in parallel with the low-sideswitch 12 _(L).

The first power converter circuit 10 further includes at least oneinductor 15 such as a choke. In the embodiment shown in FIG. 12, theindividual first converter cells 1 ₁-1 _(N1) share the inductor 15. Thatis, there is one inductor which is connected in series with low-sideswitch 12 _(L) in the first converter cell 1 ₁ and the correspondentlow-side switches in the other converter cells 1 ₂-1 _(N1). According toanother embodiment (not shown) each converter cell 1 ₁-1 _(N1) includesone inductor connected between one cell input node and the circuit nodecommon to the high-side switch and the low-side switch in the respectiveconverter cell.

Referring to FIG. 12, the first converter cell 1 ₁ further includes acontroller 14 ₁ which is configured to control operation of the low-sideswitch 12 _(L) and the high-side switch. In case the high-side switch 12_(H) is replaced with a passive rectifier element, the controller 14only controls operation of the low-side switch 12 _(L).

The low-side switch 12 _(L) receives a drive signal S12 _(L) from thecontroller 14, wherein the drive signal S12 _(L) switches the low-sideswitch 12 _(L) either on or off. Equivalently, the high-side switch 12_(H) receives a drive signal S12 _(H) from the controller 14, whereinthe drive signal S12 _(H) switches the high-side switch 12 _(H) eitheron or off. According to one embodiment, the controller 14 drives thelow-side switch 12 _(L) and the high-side switch 12 _(H) such that theyare not switched on at the same time, so as to prevent the DC linkcapacitor 11 ₁ from being discharged via these switches 12 _(L), 12_(H).

According to one embodiment, the controller 14 in the first convertercell 1 ₁ and corresponding controllers in the other converter cells 1₂-1 _(N1) are controlled by a controller 4 of the first power converter10. This controller 4 will also be referred to as main controller of thefirst power converter 10 in the following. Ways of operation of thismain controller 4 and possible implementations are explained below.

According to one embodiment, the main controller 4, via the controller14 in the first converter cell 1 ₁ and corresponding controllers in theother converter cells 1 ₂-1 _(N1) is configured to control (regulate)the total DC link voltage V2 _(TOT). According to one embodiment, themain controller 4 is further configured to control a current waveform ofthe input current I_(IN) such that the waveform of the input currentI_(IN) substantially corresponds to the waveform of the input voltageV_(IN). A phase difference between the waveform of the input voltageV_(IN) and the resulting waveform of the input current I_(IN) may bezero or may be different from zero. Controlling the input current I_(IN)to have substantially the same waveform as the input voltage V_(IN) mayhelp to control the power factor of the input power P_(IN) received atthe input IN1, IN2. A first power converter 10 configured to control thewaveform of the input current I_(IN) to substantially be equal to thewaveform of the input voltage V_(IN) will be referred to as a firstpower converter 10 with a PFC (Power Factor Correction) capability or,briefly, as a first PFC power converter 10.

One embodiment of a main controller 4 which is configured to control thetotal DC link voltage V2 _(TOT), and the current waveform of the inputcurrent I_(IN) is shown in FIG. 13. Referring to FIG. 13, the maincontroller 4 includes an input reference current controller 41 and aconverter cell controller 42, which will also be referred to asmodulation index controller. The input reference current controller 41is configured to generate an input current reference signal I_(IN) _(—)_(REF). The input current reference signal I_(IN) _(—) _(REF) representsthe desired current level (set point) of the input current I_(IN), whichis required in order to control the total DC link voltage V2 _(TOT) suchthat a voltage level of the total DC link voltage V2 _(TOT) equals apredefined voltage level. A level of this input current reference signalI_(IN) _(—) _(REF) may vary over time as the input voltage V_(IN)varies. The input reference current controller 41 receives an inputvoltage signal V_(IN) _(—) _(M) which represents the instantaneousvoltage level of the input voltage V_(IN). This input voltage signalV_(IN) _(—) _(M) may be obtained by measuring the input voltage V_(IN),or by other means. The input reference current controller 41 furtherreceives DC link voltage signals V2 ₁ _(—) _(M)-V2 _(N2) _(—) _(M). Eachof these DC link voltage signals V2 ₁ _(—) _(M)-V2 _(N2) _(—) _(M)represents one of the DC link voltages V2 ₁-V2 _(N2). These DC linkvoltage signals V2 ₁ _(—) _(M)-V2 _(N2) _(—) _(M) may be obtained bymeasuring the individual DC link voltages V2 ₁-V2 _(N2). The inputreference current controller 41 further receives a total DC link voltagereference signal V2 _(TOT) _(—) _(REF). This reference signal V2_(TOT-REF) represents the desired (predefined) voltage level of thetotal DC link voltage V2 _(TOT). The input reference current controller41 calculates the input current reference signal I_(IN) _(—) _(REF)based on these input signals. As the input reference current controller41 generates the current level of the input current reference signalI_(IN) _(—) _(REF) such that the total DC link voltage assumes thedesired level defined by the DC link voltage reference signal V2 _(TOT)_(—) _(REF), the input reference current controller 41, in the presentembodiment as well as in other embodiments explained below, may also bereferred to as DC link voltage controller.

The modulation index controller 42 receives the input current referencesignal I_(IN) _(—) _(REF) and an input current signal I_(IN) _(—) _(M).The input current signal I_(IN) _(—) _(M) represents the instantaneouscurrent level of the input current I_(IN). This input current signalI_(IN) _(—) _(M) may be obtained by measuring the input current I_(IN),or by other means. The modulation index controller 42 outputs a controlsignal m which is received by the controllers 14 ₁-14 _(N) in theindividual first converter cells 1 ₁-1 _(N). Referring to FIG. 12, eachof the controllers (more precisely, the controller in each of theconverter cells) receives a control signal m₁-m_(N1) from the maincontroller 4. According to one embodiment, the individual firstconverter cells 1 ₁-1 _(N1) receive the same control signal m, so thatm=m₁=m₂=m₃=m_(N1). Details on this control signal m, which will also bereferred to as modulation index m in the following, are explained below.Before going into further detail on the modulation index m, embodimentsof the input reference current controller 41 and the converter cellcontroller 42 are explained with reference to FIG. 14. The modulationindex controller 42 serves to control the input current I_(IN). Thus,the modulation index controller 42 may also be referred to as (input)current controller.

In context with FIG. 14, the input reference current controller 41 willbriefly be referred to as current controller. Referring to FIG. 14, thecurrent controller 41 may include an error filter 411 which receives theDC link voltage signals V2 ₁ _(—) _(M)-V2 _(N2) _(—) _(M) and the totalDC link voltage reference signal V2 _(TOT) _(—) _(REF). The error filter411 generates an error signal V2 _(ERR) which is dependent on adifference between the total DC link voltage reference signal V2 _(TOT)_(—) _(REF) and the sum of the individual DC link voltage signals V2 ₁_(—) _(M)-V2 _(N) _(—) _(M). The sum of these DC link voltage signals V2₁ _(—) _(M)-V2 _(N) _(—) _(M) represents the total DC link voltage V2_(TOT). The error filter may calculate the difference

V ₂ _(—) _(TOT)−Σ_(i=1) ^(N1) V2_(i) _(—) _(M)  (6)

and filter this difference in order to generate the error signal V2_(ERR). The filter may have one of an proportional (P) characteristic, aproportional-integral (PI) characteristic, and aproportional-integral-derivative (PID) characteristic. A multiplier 412receives the error signal V2 _(ERR) and the total DC link voltage signalV2 _(TOT) _(—) _(REF) and provides the product of these signals V2_(ERR), V2 _(TOT) _(—) _(REF) as an output signal A. An optional divider413 receives the multiplier output signal A and a signal B, wherein thesignal B is dependent on the square of the peak voltage level V_(IN)_(—) _(MAX) of the input voltage V_(IN). In the embodiment shown in FIG.14,

$\begin{matrix}{B = {\frac{V_{IN\_ MAX}^{2}}{2}.}} & (7)\end{matrix}$

An output signal C of the divider 413 equals the quotient A/B of thedivider input signals A, B. A further multiplier 414 receives thedivider output signal C and the input voltage signal V_(IN) _(—) _(M)and is configured to multiply the instantaneous levels of these signalsC and V_(IN) _(—) _(M). The further multiplier 414 provides the inputcurrent reference signal I_(IN) _(—) _(REF) as an output signal.

As explained with reference to FIG. 13, the input current referencesignal I_(IN) _(—) _(REF) defines the desired current level of the inputcurrent I_(IN). In case the input voltage V_(IN) is timely varying theinput current reference signal I_(IN) _(—) _(REF) is also timelyvarying. This is a result of generating the input current referencesignal I_(IN) _(—) _(REF) by multiplying the input voltage signal V_(IN)_(—) _(M) with the output signal C of the divider 413. The divider 413may be omitted. In this case, the further multiplier 414 receives theoutput signal A from the multiplier 412 as an input signal. Assumingthat the input current reference signal I_(IN) _(—) _(REF) is a periodicsignal with a frequency defined by the input voltage signal V_(IN) _(—)_(M), then an amplitude of the input current reference signal I_(IN)_(—) _(REF) is defined by an amplitude of the input voltage signalV_(IN) _(—) _(M), and by one of the divider output signal C and themultiplier output signal A. These signals C and A are dependent on thetotal DC link voltage V2 _(TOT). The error filter 411 is configured togenerate the error signal V2 _(ERR) such that a signal level of theerror signal V2 _(ERR) increases when the total DC link voltage V2_(TOT) falls below the level defined by the total DC link voltagereference signal V2 _(TOT) _(—) _(REF), so as to increase the level ofthe multiplier output signal A and the amplitude of the input currentreference signal I_(IN) _(—) _(REF), and so as to regulate the total DClink voltage V2 _(TOT) such that a voltage level of the total DC linkvoltage V_(TOT) substantially corresponds to the voltage level asdefined by the total DC link voltage reference signal V2 _(TOT) _(—)_(REF). Equivalently, the error filter 411 decreases a level of theerror signal V2 _(ERR) when the voltage level of the total DC linkvoltage V_(TOT) increases to above the voltage level defined by thetotal DC link voltage reference signal V2 _(TOT) _(—) _(REF), so as toreduce the amplitude of the input voltage reference signal I_(IN) _(—)_(REF) and to counteract a further increase of the total DC link voltageV2 _(TOT).

The optional divider 413 may be used in those applications, in which theamplitude of the input voltage V_(IN) may vary. The divider 413 acts inaccordance with the feed-forward principle and helps to reduce theamplitude of the input current I_(IN), by reducing the amplitude of theinput current reference signal I_(IN) _(—) _(REF), when the amplitude ofthe input voltage V_(IN) increases. In this case, the average inputpower, which is the input power averaged over one period of the inputvoltage V_(IN) is substantially independent of the amplitude of theinput voltage V_(IN) and is substantially defined by the error signal V2_(ERR) and the total DC link voltage reference signal V2 _(TOT) _(—)_(REF).

Referring to FIG. 14, the modulation index controller 42 includes afirst filter 422 which receives the input current signal I_(IN) _(—)_(M). A subtractor 421 receives the input current reference signalI_(IN) _(—) _(REF) and the filter output signal 422. This subtractor 421subtracts the instantaneous signal level of the filter output signalI_(IN) _(—) _(F) from the instantaneous level of the input currentreference signal I_(IN) _(—) _(REF). An output signal I_(IN) _(—) _(ERR)of the subtractor 421 represents a current error. That is, thesubtractor output a signal I_(IN) _(—) _(ERR) which represents aninstantaneous difference between the desired input current level and theactual input current level. A second filter 423 receives this currenterror signal I_(IN) _(—) _(ERR) and provides the modulation index m.According to one embodiment, the first filter 422 has a low-passcharacteristic. The second filter 423 may have one of a P, PI, and a PIDcharacteristic.

It can be shown that, in case the input voltage V_(IN) is a periodicvoltage such as, for example, a rectified sine voltage with a frequencyof 100 Hz or 120 Hz, the modulation index m is also a periodic signal,substantially with the same frequency as the input voltage V_(IN). FIG.15 schematically illustrates the relationship between the input voltageV_(IN) and the modulation index m. As, referring to FIG. 14, the inputcurrent reference signal I_(IN) _(—) _(REF) is obtained by multiplyingthe input voltage signal V_(IN) _(—) _(M) with one of signals C and Awhich are dependent on the total DC link voltage, the waveform shown inFIG. 15 which represents the input voltage V_(IN) also represents theinput current reference signal I_(IN) _(—) _(REF) (when assuming thatthe voltage level of the total DC link voltage V_(TOT) does not changeduring the time period illustrated in FIG. 15). Referring to FIG. 15,there may be a phase-shift φ between the input voltage V_(IN) and theinput current reference signal I_(IN) _(—) _(REF), respectively, on theone hand, and the modulation index m on the other hand. This phasedifference φ which is at most several degrees, may vary based on thedifference between the input voltage reference signal I_(IN) _(—) _(REF)and the filtered input current signal I_(IN) _(—) _(F), and on thevoltage V15 across the inductor 15 (see, FIG. 11). Further, it can beshown that an amplitude of the varying modulation index m is dependenton the amplitude of the input voltage V_(IN), whereas the amplitude ofthe modulation index m increases as the amplitude of the input voltageV_(IN) increases. According to one embodiment the main controller 4 isconfigured to generate the modulation index m as a normalized signalwith values of between 0 and 1, wherein the modulation index m has anamplitude of 1 only in those cases in which an amplitude of the inputvoltage V_(IN) corresponds to the total DC link voltage V_(TOT).

FIG. 16 shows one embodiment of the controller 14 in the first convertercell 1 ₁ shown in FIG. 12. Each of the controllers (not shown in FIG.12) in the other converter cells 1 ₂-1 _(N1) may be implemented inaccordance with the controller 14 shown in FIG. 16. Referring to FIG.16, the controller 14 ₁ is configured to calculate a duty cycle d₁ basedon the modulation index m₁ received from the cell controller 42. In theembodiment shown in FIG. 16, calculating the duty cycle d includescalculating the duty cycle d as follows:

d ₁=1−m ₁  (8).

For the purpose of explanation it is assumed that each of the firstconverter cells 1 ₁-1 _(N1) receives the same modulation index m fromthe main controller, so that in the controller of each of the firstconverter cells 1 ₁-1 _(N1) the same duty cycle d=1−m is calculated.

Like the modulation index m₁, the duty cycle d₁ may vary between 0and 1. Like the modulation index m₁, the duty cycle d₁ may timely varyand may vary between 0 and 1. A PWM controller 142 receives the dutycycle or, more precisely, a signal representing the duty cycle d₁, andgenerates the drive signal S12 _(L) for the low-side switch 12 _(L) and,optionally, the drive signal S12 _(H) for the high-side switch 12 _(H)based on the duty cycle d₁.

One way of operation of the PWM controller 142 shown in FIG. 16 isexplained with reference to FIG. 17 in which timing diagrams of thedrive signal S12 _(L) received by the low-side switch 12 _(L) and thedrive signal S12 _(H) received by the high-side switch 12 _(H) areshown. Each of these drive signal S12 _(L), S12 _(H) can assume anon-level which switches on the respective switch, and an off-level whichswitches off the respective switch. Just for the purpose of explanation,in FIG. 17, an on-level is drawn as a high signal level, and anoff-level is drawn as a low signal level.

Referring to FIG. 17, the PWM controller 142 is configured to cyclicallyswitch on the low-side switch 12 _(L). In particular, the PWM controller142 may be configured to periodically switch on the low-side switch 12_(L). In FIG. 17, Tp denotes the duration of one drive cycle of thelow-side switch 12 _(L). The time period Tp is defined by a switchingfrequency fp, with Tp=1/fp. The switching frequency fp is, for example,a frequency selected from a frequency range of between 18 kHz andseveral 100 kHz. In FIG. 17 Ton denotes an on-time of the low-sideswitch 12 _(L), which is a time period within one drive cycle in whichthe low-side switch 12 _(L) is switched on. The duty cycle d₁ definesthe duration of this on-time relative to the duration Tp of one drivecycle, wherein

d ₁ =Ton/Tp  (9).

Thus, the on-time increases relative to the time period Tp of one drivecycle as the duty cycle d₁ increases, and vice versa.

Referring to FIG. 17, the PWM controller 142 may switch on and off thehigh-side switch 12 _(H) complementary to switching on and off the lowside switch 12 _(L). That is, the PWM controller 142 may be configuredto switch on the high-side switch 12 _(H) when the low-side switch 12_(L) is switched off, and vice versa. There may be a delay time betweenswitching off the low-side switch 12 _(L) and switching on the high-sideswitch 12 _(H), and between switching off the high-side switch 12 _(H)and again switching on the low-side switch. However, such delay timesare not shown in FIG. 17. During such delay times, the rectifier elementof the high-side switch 12 _(H) conducts. In case the high-side switch12 _(H) is replaced by a rectifier element, the rectifier element“automatically” conducts when the low-side switch 12 _(L) is in the offstate.

FIG. 18 shows one embodiment of the PWM controller 142 in the controller14 shown in FIG. 16. Referring to FIG. 18, the PWM controller 142 mayinclude a clock generator 143 which generates a first clock signal CLK1.A frequency of this first clock signal CLK1 may be higher than theswitching frequency fp. According to one embodiment, the frequency ofthe first clock signal CLK1 is at least several MHz. A frequency divider144, which may be implemented with a counter or the like, receives thefirst clock signal CLK1 and generates a second clock signal CLK2. Thesecond clock signal CLK2 defines the switching frequency fp. This secondclock signal CLK2 is also illustrated in FIG. 17. Referring to FIG. 17,the drive signal S12 _(L) of the low-side switch 12 _(L) may assume anon-level each time a signal pulse of the second clock signal CLK2occurs. A latch, such as an SR flip flop 145 may receive the secondclock signal CLK2 at a set input S. A first driver 146 has an inputcoupled to a noninverting first output Q of the flip flop 145 andgenerates the drive signal S12 _(L) of the low-side switch 12 _(L) basedon the output signal at the first input Q of the flip flop 145. Anoptional second driver 147 generates the drive signal S12 _(H) of thehigh-side switch 12 _(H) based on an output signal at a second invertingoutput Q′ of the flip flop 145. In order to adjust the on-time Ton ofthe low-side switch 12 _(L), a timer 148 receives the second clocksignal CLK2, the duty cycle signal d and the first clock signal CLK1.The timer 145 is configured to reset the flip flop 145, in order tocause the drive signal S12 _(L) to assume the off-level at a predefinedtime period after a signal pulse of the second clock signal CLK2,wherein this time period is defined by the duty cycle d.

It should be noted that FIG. 18 shows only one of a variety of possibleimplementations of the PWM controller 142. Of course, the implementationof the PWM controller 142 is not restricted to the specific embodimentshown in FIG. 18.

It can be shown that the modulation index generated as explained aboveapproximately corresponds to

m=V _(IN) /V2_(TOT)  (10).

wherein V_(IN) denotes the instantaneous voltage level of the inputvoltage V_(IN), and V2 _(TOT) represents the (desired) total DC linkvoltage. However, this is only an approximation. Referring to what isstated in context with FIGS. 13 and 14 above, the modulation index m isnot only dependent on the input voltage V_(IN), but may further varybased on the difference between current level of the input currentI_(IN) and the reference input current I_(IN) _(—) _(REF).

According to one embodiment, the controller 14 in the first convertercell 1 ₁ and corresponding controllers in the other converter cells 1₂-1 _(N1) receive the same modulation index m from the main controller4, and the individual converter cells 1 ₁-1 _(N1) are operated in aninterleaved fashion. This is explained with reference to FIGS. 19A and19B. FIGS. 19A and 19B show timing diagrams of the drive signal S12 _(L)of the low-side switch 12 _(L) in the first converter cell 1 ₁ andtiming diagrams of drive signals S12 _(L2)-S12 _(LN1) of correspondinglow-side switches in the other converter cells 1 ₂-1 _(N1). In FIGS. 19Aand 19B, these drive signals S12 _(L)-S12 _(LN1) are shown at twodifferent duty cycles d, namely d=0.625 in FIG. 19A and d=0.125 in FIG.19B. Operating the individual converter cells 1 ₁-1 _(N1) in aninterleaved fashion means that the drive cycles of the individualconverter cells 1 ₁-1 _(N1) begin with a time offset of Tp/N1. Here,like in the embodiments explained before, N1 denotes the number of firstconverter cells 1 ₁-1 _(N1). If, for example, N1=4, the time offset isTp/4, as shown in FIGS. 19A and 19B. For example, there is a delay timeof Tp/4 between the begin of the on-time of the drive signal S12 _(L) inconverter cell 1 ₁ and the begin of the on time of the drive signal S12_(L2) in the converter cell 1 ₂, there is a delay time Tp/4 between thebegin of the—on time of the drive signal S12 _(L2) in the converter cell1 ₂ and the begin of the on time of the drive signal S12 _(L3) in theconverter cell 1 ₃, and there is a delay time Tp/4 between the begin ofthe on time of the drive signal S12 _(L3) in the converter cell 1 ₃ andthe begin of the on time of the drive signal S12 _(LN1) in the convertercell 1 _(N1). Operating the individual converter cells 1 ₁-1 _(N1) in aninterleaved fashion results in an overall switching frequency of N1·fp.This higher overall switching frequency may help to reduce ripples inthe input current I_(IN) that may result from the switched-modeoperation of the first power converter 10 or, more precisely, from aswitched mode operation of the individual first converter cells 1 ₁-1_(N1).

Referring to FIG. 12 and the corresponding description a current levelof the input current I_(IN) can be adjusted by modulating a voltage V15across the inductor 15. The voltage level of this voltage V15 isdependent on the instantaneous value of the input voltage V_(IN), the DClink voltages V2 ₁-V2 _(N2) and the operation states of the individualfirst converter cells 1 ₁-1 _(N1). For the purpose of explanation it isassumed that the individual DC link voltages V2 ₁-V2 _(N2) aresubstantially equal and the number N2 of DC link capacitors equals thenumber N1 of first converter cells 1 ₁-1 _(N1) (N1=N2). In this case,each of these DC link voltages V2 ₁-V2 _(N2) equals V2 _(TOT)/N1.Further, it is assumed that each converter cell 1 ₁-1 _(N1) can assumean on-state, which is an operation state in which the respectivelow-side switch 12 _(L) is switched on, and an off-state in which therespective low-side switch 12 _(L) is switched off. Thus, in the timingdiagrams shown in FIGS. 19A and 19B, the on-times of the low-side switchdrive signals I2 _(L)-12 _(LN1) represent the on-times of the individualfirst converter cells.

Assuming that electrical resistances of the individual low-side switches(from which in FIG. 12 only the low-side switch 12 _(L) of the firstconverter cell 1 ₁ is shown) in the first converter cells 1 ₁-1 _(N) canbe neglected, the cell input voltage V1 ₁-V1 _(N1) at the cell input ofthe individual converter 1 ₁-1 _(N1) cells is zero when the convertercell 1 ₁-1 _(N1) is in the on-state, and equals the DC link voltage (V2_(TOT)/N1) of the respective converter cell when the converter cell isin the off-state. The inductor voltage V15 is given by

V15=V _(IN) −V1_(TOT)  (11),

wherein V1 _(TOT) represents the overall voltage at the cell inputs ofthe individual first converter cells. That is

V ₁ _(—) _(TOT)=Σ_(i=1) ^(N1) V1_(i)  (12).

In case each of the first converter cells 1 ₁-1 _(N1) includes aninductor (not shown), the cell input voltages V1 ₁-V1 _(N1) are thevoltages across the individual low-side switches. V15 is then the totalvoltage across the plurality of inductors.

Operating (driving) the individual first converter cells 1 ₁-1 _(N1)based on the modulation index m as explained with reference to FIGS.13-19B herein before, causes the inductor voltage V15 substantially tovary between V_(IN)−(k·V2 _(TOT)/N1) and V_(IN)−((k+1)·V2 _(TOT)/N1),wherein k is dependent on the modulation index m. and equals the numberof first converter cells that are in the off-state at one time. k may beobtained by

$\begin{matrix}{{k = {{{Round}\left\lbrack \frac{{V_{IN} \cdot N}\; 1}{V\; 2_{TOT}} \right\rbrack} = {{Round}\left\lbrack {{m \cdot N}\; 1} \right\rbrack}}},} & (13)\end{matrix}$

where Round[·] is a mathematic function that rounds the result of theoperation in the square brackets to the next lower integer, V_(IN) isthe instantaneous level of the input voltage V_(IN), and m is themodulation index. If, for example, the instantaneous level of the inputvoltage V_(IN) is below the level of one DC link voltage (V2 _(TOT)/N1),then k=0 so that the total cell input voltage V1 _(TOT) varies between 0and V2 _(TOT)/N1 until the input voltage V_(IN) reaches V2 _(TOT)/N1. Inthis way, the total cell input voltage V1 _(TOT) “follows” theinstantaneous value of the input voltage V_(IN). In other words, theconverter cells 1 ₁-1 _(N1) generate (modulate) the total cell inputvoltage V1 _(TOT) such that the total cell input voltage V1 _(TOT)“tracks” the input voltage V_(IN). In this way, the voltage V15 acrossthe inductor 15 can be controlled. This is explained with reference toFIGS. 12 and 19A in the following.

In the embodiment shown in FIG. 19A, the individual converter cells 1₁-1 _(N1) are operated with a duty cycle of d=0.625. In this embodiment,the modulation index m is 0.375 which indicates that the instantaneousvalue of the input voltage V_(IN) is relatively low as compared to the(desired) overall DC link voltage V2 _(TOT). Referring to the equationabove, k=1 when m=0.375 and when there are N1=4 converter cells(k=Round[0.375·4]=Round[1.5]=1) so that at m=0.375 the total cell inputvoltage V1 _(TOT) changes between V2 _(TOT)/N1 and 2·V2 _(TOT)/N1. Thatis, either one or two converter cells are in the off-state at one time,and either three or two converter cells are in the off-state at the sametime, respectively. If three first converter cells 1 ₁-1 _(N1) are inthe on-state, the total input cell voltage V1 _(TOT) is (N1−3). V2_(TOT)/N1. That is, in this specific embodiment with N1=4, the totalinput cell voltage V1 _(TOT) is V2 _(TOT)/N1. If two of the convertercells 1 ₁-1 _(N) are operated in the on-state, the total cell inputvoltage V1 _(TOT) is (N1−2)·V2 _(TOT)/N1. The inductor voltage V15 inthese two cases is

V15=V _(IN)−(N1−3)·V2_(TOT) /N1  (14A)

V15=V _(IN)−(N1−2)·V2_(TOT) /N1  (14B).

A modulation index m=0.375 indicates that the instantaneous value of theinput voltage V_(IN) substantially corresponds to the 0.375·V2 _(TOT),so that the inductor voltage V15 is positive when three of the firstconverter cells 1 ₁-1 _(N) are in the on-state, and negative when two ofthe first converter cells 1 ₁-1 _(N) are in the on-state. Thus, in thefirst case, the inductor current I_(IN) increases, while, in the secondcase, it decreases. In those time periods in which the total cell inputvoltage V1 _(TOT) is lower that the instantaneous value of the inputvoltage V_(IN), energy is inductively stored in the inductor 15, and inthose time periods in which the instantaneous voltage level of the inputvoltage V_(IN) is below the total cell input voltage V1 _(TOT), theenergy stored in the inductor 15 is transferred to the DC linkcapacitors of those first converter cells 1 ₁-1 _(N1) which are in theoff-state. As each of the first converter cells 1 ₁-1 _(N1) is switchedon and off in one drive cycle, the DC link capacitors 11 ₁-11 _(N2) ofthe individual first converter cells 1 ₁-1 _(N1) are equally chargedwhen the individual first converter cells 1 ₁-1 _(N1) receive the samemodulation index m by the main converter 4.

Referring to the embodiment shown in FIG. 19B, a duty cycle d=0.125corresponds to a modulation index of m=0.875. In this case, theinstantaneous voltage level of the input voltage V_(IN) is close to thetotal DC link voltage V2 _(TOT). At m=0.875 and N1=4, k=3 so that threeor four converter cells are in the off-state at one time. Consequently,the total cell input voltage V1 _(TOT) varies between (N1−1)·V2_(TOT)/N1 if three of the first converter cells 1 ₁-1 _(N1) are in theoff-state (only one is in the on-state), and V2 _(TOT) if each of thefirst converter cells 1 ₁-1 _(N1) is in the off-state (none is in theon-state).

FIG. 20 schematically illustrates one period of the input voltage V_(IN)and the total cell input voltage V1 _(TOT) during this one period of theinput voltage V_(IN). The embodiment shown in FIG. 20 is based on afirst power converter 10 with N1=4 first converter cells 1 ₁-1 _(N1) andN2=4 DC link capacitors 11 ₁-11 _(N2). As can be seen from FIG. 20,dependent on the instantaneous voltage level of the input voltage V_(IN)the total cell input voltage V1 _(TOT) switches between two voltagelevels. The difference between these two voltage levels substantially isV2 _(TOT)/N1. In FIG. 20, the dashed lines mark those instantaneousvoltage levels of the input voltage V_(IN) at which the two levelschange between which the total cell input voltage V1 _(TOT) switches.The duty cycles d and the modulation indexes m associated with thoseinstantaneous voltage levels of the input voltage V_(IN) marked by thedashed lines are also shown in FIG. 20. It should be noted that thewaveform of the total DC link voltage shown in FIG. 20 can be obtainedby operating the individual converter cells 1 ₁-1 _(N1) with the same(or substantially the same) modulation index m. However, referring tothe explanation further below, it is also possible to operate theindividual converter cells 1 ₁-1 _(N1) with different modulation indicesand to obtain a waveform as shown in FIG. 20.

FIG. 21 shows one example of how the controller 14 in the firstconverter cell 1 ₁ and corresponding controllers in the other convertercells 1 ₂-1 _(N1) may be synchronized in order for the controllers tooperate the individual first converter cells 1 ₁-1 _(N1) in aninterleaved fashion, as explained with reference to FIGS. 19A and 19B.In FIG. 21, reference character 14 denotes the controller in the firstconverter cells 1 ₁, as shown in FIG. 12, and reference characters 14₂-14 _(N1) denote corresponding controllers in the other first convertercells 1 ₂-1 _(N1). In the embodiments shown in FIGS. 19A and 19B, thedrive cycles of the individual first converter cells 1 ₁-1 _(N1) startin a predefined order. In this case, the individual controllers can besynchronized as shown in FIG. 21. In this embodiment, controller 14 offirst converter cell 1 ₁ forwards the second clock signal CLK2 (which,in the first converter cell 1 ₁, is used to define the beginning of theon-time) to controller 14 ₂ of the first converter cell 1 ₂ which,referring to FIGS. 19A and 19B, is next in order to start thecorresponding drive cycle. The controller 14 ₂ forward its second clocksignal CLK2 ₂ (which, in the first converter cell 1 ₂, is used to definethe beginning of the on-time) to controller 14 ₃, which forwards itssecond clock signal CLK2 ₃ (which, in the first converter cell 1 ₃, isused to define the beginning of the on-time) to the controller 14 _(N1).The second clock signals CLK2-CLK2 ₃ are transmitted from one controllerto the other controller via isolation barriers 16 ₁-16 ₃ which mayinclude transformers, optocouplers, or the like, and which galvanicallyisolate the controllers 14-14 _(N1).

If the individual controllers 14 ₁-14 _(N1) are synchronized as shown inFIG. 21, the controller 14 of the first converter cell 1 ₁ may beimplemented as explained with reference to FIGS. 16 and 18. The PWMcontrollers 142 in the other controllers 14 ₂-14 _(N1) may beimplemented as shown in FIG. 22. The PWM controller 142 shown in FIG. 22is a modification of the PWM controller 142 shown in FIG. 18. The PWMcontroller shown in FIG. 22 is different from the one shown in FIG. 18in that instead of the frequency divider 144 there is a further delayelement 149 which receives the second clock signal CLK2 _(i-1) fromanother controller and generates the second clock signal CLK2 _(i) basedon the received second clock signal CLK2 _(i-1) and the desired timeoffset (Tp/4 in FIGS. 19A and 19B) between the drive cycles of theindividual converter cells 1 ₁-1 _(N1). In FIG. 22 CLK2 _(i-1) denotesthe second clock signal received by the respective controller. If, forexample, the PWM controller shown in FIG. 22 is the PWM controller ofthe controller 14 ₃ shown in FIG. 21, then CLK2 _(i-1) is the clocksignal CLK2 received from the controller 14 ₂, and CLK₂ is the controlsignal used in the converter cell 1 ₂ to control the beginning and theend of the on-time.

The interleaved operation of the first converter cells 1 ₁-1 _(N1)explained before in which the individual converter cells are operated atthe same duty cycle is only one way of operating the series connectedfirst converter cells 1 ₁-1 _(N1). In this embodiment, each of theconverter cells is operated in a PWM fashion (at the switching frequencyfp) such that each converter cell is in the on-state for a certain timeperiod and in the off state for a certain time period in each drivecycle. That is, the individual converter cells are operated in the sameoperation mode. According to another embodiment, in one drive cycle,only one of the first converter cells 1 ₁-1 _(N1) is operated in a PWMfashion based on the modulation index, while the other first convertercells are either in the on-state or in the off-state throughout thecomplete duration of one drive cycle. Thus, each of the converter cellsis operated in one of three different operation modes, the PWM mode, theon-state (on-mode), and the off-state (off-mode). An on-state of oneconverter cell throughout one drive cycle corresponds to a duty cycle of1 (and a modulation index of 0) of the corresponding converter cell, andan off-state of one converter cell throughout one drive cyclecorresponds to a duty cycle of 0 (and a modulation index of 1) of thecorresponding converter cell. That is, operating one cell in a PWMfashion and operating the other cells either in the on-state or theoff-state corresponds to operating the individual cells at differentduty cycles and modulation indices, respectively. In general, themodulation indices m₁-m_(N1) of the individual converter cells 1 ₁-1_(N1) are selected such that

V _(IN)=Σ_(i=1) ^(N1) m _(i) ·V2_(i) =m·V2_(TOT)  (15),

where N1=N2, V_(IN) is the instantaneous level of the input voltageV_(IN), m_(i) is the modulation index of one converter cell, V2 _(i) isthe corresponding DC link voltage, m is the overall modulation index ofthe power converter, and V2 _(TOT) the voltage level of the total DClink voltage. If the individual DC link voltages V2 ₁-V2 _(N2) aresubstantially equal and equal to V2 _(TOT)/N1 then

$\begin{matrix}{{V_{IN} = {{\frac{V\; 2_{TOT}}{N\; 1} \cdot \left( {\sum\limits_{i = 1}^{N\; 1}m_{i}} \right)} = {{m \cdot V}\; 2_{TOT}}}},{and}} & (16) \\{\left( {\sum\limits_{i = 1}^{N\; 1}m_{i}} \right) = {N\; {1 \cdot {m.}}}} & (17)\end{matrix}$

Operating the individual converter cells at different modulation indicesis explained with reference to FIG. 23. FIG. 23 shows timing diagrams ofthe drive signals S12 _(L)-12 _(LN1) of the low-side switches in theindividual first converter cells, wherein the signal levels of the drivesignals S12 _(L)-12 _(LN1) represent the operation state of theindividual first converter cells 1 ₁-1 _(N1), as explained above.

For the purpose of explanation it is assumed that m=0.625 and N1=4. As4.0.625=2.5=1+1+0+0.5, an overall modulation index of m=0.625 of thepower converter 10 can be obtained by operating two converter cells at amodulation index of 1 (at a duty cycle of 0), one converter cell at amodulation index of 0 (at a duty cycle of 1) and one converter cell at amodulation index of 0.5 (at a duty cycle of 0.5). This is illustrated inFIG. 23. In a first drive cycle shown in FIG. 23, m₁=0.5, m₂=m₃=1, andm_(N1)=0, that is, converter cell is operated in a PWM fashion at a dutycycle of d₁=0.5 (=1−m₁=1−0.5), converter cells 1 ₂ and 1 ₃ are in theoff-state, and converter cell 1 _(N1) is in the on-state. In a nextdrive cycle, the modulation indices of 1, 1, 0, and 0.5 may be assignedto the converter cells in another way (as illustrated in FIG. 23).However, it is also possible to operate each of the converter cells withthe same modulation index for several drive cycles.

The overall duty cycle of the first power converter 10, such as the dutycycle d=0.375 in FIG. 23 denotes the average duty cycle of each of thefirst converter cells, that is

$\begin{matrix}{{d = \frac{\sum\limits_{i = 1}^{N\; 1}d_{i}}{N\; 1}},} & (18)\end{matrix}$

wherein d_(i) denotes the individual duty cycle of each first convertercell. In the embodiments shown in FIGS. 19A and 19B, the individualconverter cells have the same duty cycle and the same modulation indexwhich is the overall duty cycle and the overall modulation index,respectively.

FIG. 24 shows one embodiment of a converter cell 1 _(i) which may beused in a multi-cell converter with IS topology of the type shown inFIG. 12 when the multi-cell converter receives a sinusoidal voltage asthe input voltage V_(IN). That is, each of the converter cells shown inFIG. 12 may be replaced with a converter cell of the type shown in FIG.24. In FIG. 24, V1 _(i) denotes the cell input voltage, V2 _(i) denotesthe DC link voltage at the associated DC link capacitor 11 _(i), I1 _(i)denotes the cell output current (which is the current into the circuitnode to which the DC link capacitor 11 _(i) is connected thereto).

Referring to FIG. 24, the converter cell 1 _(i) includes a bridgecircuit with two half-bridges 17, 18. Each half-bridge 17, 18 includes ahigh-side switch 17 _(H), 18 _(H) and a low-side switch 17 _(L), 18_(L). Load paths of the high-side switch 17 _(H), 18 _(H) and thelow-side switch 17 _(L), 18 _(L) of each half-bridge 17, 18 areconnected in series, whereas these series circuits are each connected inparallel with DC link capacitor 11 _(i). Each half-bridge 17, 18includes a tap, which is a circuit node common to the load paths of thehigh-side switch 17 _(H), 18 _(H), and the low-side switch 17 _(L), 18_(L) of the respective half-bridge 17, 18. A first cell input node ofthe first converter cell 1 _(i) is connected to the tap of the firsthalf-bridge 17, and a second cell input node of the first converter cell1 _(i) is connected to the tap of the second half-bridge 18. Thetopology shown in FIG. 24 will be referred to as full-bridge topology inthe following.

A first converter 10 with an IS topology and implemented with firstconverter cells of the type shown in FIG. 24 can directly process a sinevoltage provided from a power grid so that a rectifier circuit 100 (seeFIG. 11), which may cause losses, is not required. There are severalways to operate the converter cell 1 _(i) with the full-bridge topology.Two of these ways of operation are explained with reference to FIGS. 25Aand 25B below. In each of these FIGS. 25A and 25B timing diagrams of theinput voltage V_(IN) during one period of the input voltage V_(IN), andof drive signals S17 _(H)-S18 _(L) of the high-side and low-sideswitches 17 _(H)-18 _(L) are schematically illustrated.

Referring to FIG. 25A, the converter cell 1 _(i) operates differently inthe positive half-wave and the negative half-wave of the sinusoidalinput voltage V_(IN). However, within each halfwave the operation of theconverter cell 1 _(i) is very similar to the operation of one of theconverter cells 1 ₁-1 _(N) shown in FIG. 12 which each include oneelectronic switch and one rectifier element. During each half-wave, thetwo switches of one of the two half-bridges 17, 18 are operated in a PWMfashion, while the two switches of the other of the two half-bridges arein a predefined operation state during the duration of the halfwave.That is, the two switches of the one half-bridge are switched at theswitching frequency fp explained above, while the two switches of theother half-bridge are switched only once in (at the beginning of) onehalfwave. During the positive halfwave of the input voltage V_(IN) thehigh-side switch 18 _(H) of the second half-bridge 18 is in theoff-state, and the low-side switch 18 _(L) is in the on-state. Duringthis positive halfwave, the low-side switch 17 _(L) of the firsthalf-bridge 17 operates in a PWM mode, like the switching element 12 inthe converters 1 ₁ shown in FIG. 12, and the high-side switch 17 _(H) ofthe first half-bridge 17 operates like the high-side switch (rectifierelement) 13 shown in FIG. 12. That is, the high-side switch 17 _(H) alsooperates in a PWM fashion, but complementary to the low-side switch 17_(L). During the negative halfwave of the input voltage V_(IN), thehigh-side switch 17 _(H) of the first half-bridge 17 is in theoff-state, and the low-side switch 17 _(L) of the first half-bridge 17is in the on-state. The low-side switch 18 _(L) of the secondhalf-bridge 18 is operated in the PWM fashion, like the switchingelement 12 of the first converter 1 ₁ shown in FIG. 12. The high-sideswitch 18 _(H) operates like the high-side switch 13 shown in FIG. 12.That is, the high-side switch operates in a PWM fashion complementary tothe low side switch. By operating the switches of one half-bridgecomplementary in the PWM mode the two switches are not switched on atthe same time. In this embodiment, the two high-side switches 17 _(H),18 _(H) may be replaced by rectifier elements such as, for example,diodes.

In the embodiment shown in FIG. 25A, the first half-bridge 17 operatesin a PWM fashion in one halfwave (the positive halfwave in thisembodiment), and the second half-bridge 18 operates in a PWM fashion inthe other halfwave (the negative halfwave in this embodiment). Inanother way of operation explained with reference to FIG. 25B only oneof the two half-bridges 17, 18 operates in a PWM fashion while the otherhalf-bridge operates at the frequency of the input voltage V_(IN) sothat the other half-bridge switches only once in each halfwave. This wayof operation will be referred to as totem pole modulation in thefollowing. The totem pole modulation allows to optimize the half-bridgeoperated in the PWM mode in terms of low switching losses, and tooptimize the other half-bridge in terms of low conduction losses. Justfor the purpose of explanation it is assumed that the first half-bride17 operates in the PWM mode, where the switching frequency may be 18 kHzor higher, and that the second half-bridge 18 operates at twice thefrequency of the input voltage V_(IN).

Referring to FIG. 25B, in the positive halfwave the converter cell 1_(i) operates as explained with reference to FIG. 25A above. That is,the low-side switch 17 _(L) operates in a PWM fashion based on themodulation index m_(i) and the duty-cycle d_(i) (=1−m_(i)),respectively, of the converter cell 1 _(i), and the high-side switch 17_(H) switches complementarily. The high-side switch 18 _(H) of thesecond half-bridge 18 is off, and the corresponding low-side switch 18_(L) is on. In the negative halfwave, the drive schemes of theindividual switches are “inverted” as compared to the positive halfwave.That is, the high-side switch 17 _(H) operates in a PWM fashion based onthe modulation index m_(i) and the duty-cycle d_(i), respectively, ofthe converter cell 1 _(i), and the low-side switch 17 _(L) switchescomplementarily. The high-side switch 18 _(H) of the second half-bridge18 is on, and the corresponding low-side switch 18 _(L) is off.

Referring to FIG. 24, a controller 19 controls the operation of thehalf-bridges 17, 18. This controller 19 generates the drive signals S17_(H), S17 _(L), S18 _(H), S18 _(L) for the individual high-side andlow-side switches 17 _(H)-18 _(L). Like the controller 14 explained withreference to FIG. 12 above before, the controller 19 controls theindividual switches 17 _(H)-18 _(L) based on the modulation index m_(i)received by the main controller 4. The main controller 14 may beimplemented as explained with reference to FIGS. 13 and 14. When theinput voltage V_(IN) is an alternating voltage, such as sine voltageshown in FIG. 23, the modulation signal m and m_(i), respectively,generated by the main controller is an alternating signal which may varybetween −1 and +1.

FIGS. 26A and 26B show two embodiments of a controller 19 which isconfigured to control the half-bridges 17, 18 in the converter cell 1_(i) shown in FIG. 24 based on the modulation index m. FIG. 26A showsone embodiment of a controller configured to control the two half-bridgein accordance with the modulation scheme shown in FIG. 25A, and FIG. 26Bshows one embodiment of a controller configured to control the twohalf-bridges 17, 18 in accordance with the modulation scheme shown inFIG. 25B.

Referring to FIG. 26A, the controller 19 includes a first PWM controller191 which receives a first duty cycle signal d17 and drives thehigh-side switch 17 _(H) and the low-side switch 17 _(L) of the firsthalf-bridge 17 based on this first duty cycle d17. The controller 19further includes a second PWM controller 192 which receives a secondduty cycle signal d18 and is configured to drive the high-side switch 18_(H) and the low-side switch 18 _(L) of the second half-bridge 18 basedon the second duty cycle d18. The controller 19 is configured togenerate the first and second duty cycles d17, d18 as follows:

d17=1−m _(i), if m _(i)>0  (19A)

d17=1, if m _(i)≦0  (19B)

d18=1+m _(i), if m _(i)<0  (19C)

d18=1, if m _(i)≧0  (19D).

Thus, during the positive halfwave of the input voltage V_(IN) and thepositive halfwave of the modulation index (which is substantially inphase with the input voltage V_(IN)) the low-side switch 18 _(L) is on(d18=1), the high-side switch 18 _(H) is off, the low-side switch 17_(L) of the first half-bridge 17 is switched on and off with the dutycycle d17 defined by the modulation index m_(i), and the high-sideswitch 17 _(H) is switched on and off complementary to the low-sideswitch 17 _(L). During the negative halfwave, the low-side switch 17_(L) of the first half-bridge 17 is on (d17=1), the high-side switch 17_(H) is off, the low-side switch 18 _(L) of the second half-bridge isswitched on and off with the duty cycle d18 as defined by the modulationindex m_(i), and the high-side switch 18 _(H) is switched on and offcomplementary to the low-side switch 18 _(L).

The first duty cycle d17 may be generated by multiplying the modulationindex m_(i) with −1 by a first multiplier 193, adding +1 to the resultby an adder connected downstream the first multiplier 193, and bylimiting the output signal of the adder 194 to a range of between 0 and+1 by a limiter 195. The first duty cycle d17 is available at the outputof the limiter 195. The second duty cycle d18 may be generated by addingone to the modulation signal m_(i) by a second adder 196, and bylimiting an output signal of the second adder 196 to a range of between0 and 1 by a second limiter 197. The second duty cycle d18 is availableat the output of the second limiter 197.

The controller 19 shown in FIG. 26B is configured to generate the firstand second duty cycles d17, d18 as follows:

d17=1−m _(i), if m _(i)>0  (20A)

d17=−m _(i), if m _(i)≦0  (20B)

d18=0, if m _(i)<0  (20C)

d18=1, if m _(i)≧0  (20D).

Thus, during the positive halfwave of the input voltage V_(IN) and thepositive halfwave of the modulation index (which is substantially inphase with the input voltage V_(IN)) the low-side switch 18 _(L) is on(d18=1), the high-side switch 18 _(H) is off, the low-side switch 17_(L) of the first half-bridge 17 is switched on and off with the dutycycle d17 defined by the modulation index m, and the high-side switch 17_(H) is switched on and off complementary to the low-side switch 17_(L). During the negative halfwave, the low-side switch 18 _(L) is off(d18=0), the high-side switch 18 _(H) is on, the high-side switch 17_(H) of the first half-bridge 17 is switched on and off with the dutycycle d17 defined by the modulation index m_(i), and the low-side switch17 _(L) is switched on and off complementary to the high-side switch 17_(L).

The second duty cycle d18 may be generated by simply detecting thepolarity of the modulation index m_(i) using a threshold detector 198which compares the modulation index with 0. The second duty cycle d18,which is available at the output of the threshold detector 198, is 1 ifthe duty cycle m1 is above 0 and is 0 if the modulation index m_(i) isbelow 0. The first duty cycle can be obtained by subtracting themodulation index m_(i) from the output of the first threshold detector,that is from the second duty cycle using a subtractor. That is,d17=1−d18 in this embodiment. Each of first and second PWM controllers191, 192 shown in FIGS. 26A and 26B can be implemented like the PWMcontroller 142 explained with reference to FIGS. 18 and 22 hereinbefore.In case of the PWM controller 191, the duty cycle d₁ shown in FIG. 18corresponds to the first duty cycle d17, the drive signal S12 _(L) shownin FIG. 18 corresponds to the drive signal S17 _(L) of the low-sideswitch and the drive signal S12 _(H) corresponds to the drive signal S17_(H) of the high-side switch. Equivalently in case of the second PWMcontroller 192, the duty cycle d₁ shown in FIG. 18, corresponds to thesecond duty cycle d18, the drive signal S12 _(L) corresponds to thedrive signal S18 _(L) of the low-side switch, and the drive signal S12_(H) corresponds to the drive signal S18 _(H) of the high-side switch.

Referring to the explanation before, a first power converter 10 whichreceives an alternating input voltage and which includes first convertercells 1 ₁-1 _(N1) of the type explained with reference to FIGS. 24 to 26in the positive halfwave of the input voltage V_(IN) operates like thefirst power converter 10 shown in FIG. 12, and in the negative halfwave,of the input voltage V_(IN) operates in a similar way, wherein duringthe negative halfwave the first converter cells connect the DC linkcapacitors, such as DC link capacitor 11 _(i) shown in FIG. 24, to thecell input in such a way that the cell input voltage, such as voltage V1_(i) shown in FIG. 24 is negative.

One way of operation of the first power converter 10 during one periodof the input voltage V_(IN) is illustrated in FIG. 27. During thepositive halfwave, the operation is as explained with reference to FIG.20. During the negative halfwave of the input voltage V_(IN), the totalcell input voltage V1 _(TOT) varies between negative voltage levels,wherein a difference between two of these voltage levels is V2_(TOT)/N2. During the negative halfwave, the input current referencesignal I_(IN) _(—) _(REF) and, consequently, the input current I_(IN)are negative. However, the DC link voltages V2 ₁-V2 _(N2) are positive.The individual converter cells 1 ₁-1 _(N1) may operate in the sameoperation mode, as explained with reference to FIGS. 19A and 19B, or indifferent modes, as explained with reference to FIG. 23.

A first power converter 10 with an IS topology is not restricted toreceive a rectified sine voltage or a sine voltage as the input voltageV_(IN). The power converter 10 could also be operated with a directvoltage as the input voltage V_(IN). In this case, the first powerconverter generates a plurality of DC link voltages 11 ₁-11 _(N2) whichmay each have a voltage level lower than a voltage level of the inputvoltage V_(IN). Nevertheless, the level of the overall DC link voltageV2 _(TOT) may be higher than the voltage level of the input voltage. Thewaveform of a direct voltage as the input voltage V_(IN) isschematically illustrated in FIG. 2C. A multi-cell converter implementedwith converter cells 1 ₁-1 _(N1) shown in FIG. 12 may receive a positivevoltage as the input voltage V_(IN), and a multi-cell converterimplemented with converter cells shown in FIG. 24 may either receive apositive voltage or a negative voltage as an input voltage.

In case the first power converter 10 is only operated with a directvoltage as the input voltage V_(IN), the main controller 4 may besimplified as shown in FIG. 28. The main controller 4 shown in FIG. 29is based on the main controller shown in FIG. 14 and is different fromthis main controller shown in FIG. 14 in that the further multiplier 414is omitted. The input current reference signal I_(IN) _(—) _(REF)corresponds to the output signal A of the multiplier 412 or the outputsignal C of the optional divider 413, respectively. In this embodiment,the input signal B of the optional divider is V_(IN) _(—) _(MAX), whichdenotes the voltage level of the input voltage V_(IN).

FIG. 29 shows one embodiment of a multi-cell converter 10 in which theindividual converter cells 1 ₁-1 _(N1) have their cell inputs connectedin parallel at the input IN1, IN2 of the multi-cell converter. That is,each converter cell 1 ₁-1 _(N1) has a first cell input node connected tothe first input node IN1, and each converter cell 1 ₁-1 _(N1) has asecond cell input node connected to the second input node IN2 so thateach of the converter cells 1 ₁-1 _(N1) receives the input voltageV_(IN). The topology of the multi-cell converter shown in FIG. 29 willbe referred to as IP (Input Parallel) topology in the following.

In the embodiment shown in FIG. 29, the converter cells 1 ₁-1 _(N1) areimplemented with full-bridge topology, wherein only converter cell 1 ₁is shown in detail. However, a boost converter topology as shown in FIG.12 may be used as well. The multi-cell converter with the IP topologyshown in FIG. 29 is different from the multi-cell converter with the IStopology shown in FIG. 12 in that in the converter shown in FIG. 29 eachof the plurality of converter cells 1 ₁-1 _(N1) includes an inductor. Asshown in cell 1 ₁, the inductor 15 ₁ in each cell is connected betweenone cell input node, such as the first cell input node, and the bridgecircuit with the two half-bridges 17, 18. Cell input voltages V1₁-V_(1N1) of those converter cells are the voltages between the taps ofthe half-bridges. This is in correspondence with the converter cell 1_(i) shown in FIG. 24.

In the multi-cell power converter 10 with the IP topology shown in FIG.29, each first converter cell 1 ₁-1 _(N1) is configured to control(regulates) its DC link voltage V2 ₁-V2 _(N1). For this, each of theseconverter cells 1 ₁-1 _(N1) includes a controller, wherein in FIG. 29only the controller 41 of the converter cell 1 ₁ is shown. Each of thesecontrollers may be implemented in accordance with the main controller 4shown in FIGS. 13 and 14, with the difference that the controllers inthe individual converter cells 1 ₁-1 _(N1) do not receive signalsrepresenting each of the DC link voltages V2 ₁-V2 _(N2), but only asignal representing the DC link voltage of the respective convertercell, and a signal representing the desired level of the converter cell.One embodiment of the controller 4 ₁ in the converter cell 1 ₁ is shownin FIG. 30. The controllers in the other converter cells may beimplemented equivalently.

The controller 4 ₁ shown in FIG. 30 is based on the main controller 4shown in FIG. 14 and is different from the main controller shown in FIG.14 in that it outputs the modulation index m_(i) only for one convertercell 1 ₁. Further, this modulation index m_(i) is calculated based onthe DC link voltage signal V₂₁ _(—) _(M) of the respective convertercell, the DC link voltage reference signal V2 ₁ _(—) _(REF) of therespective converter cell, and, optionally, on the instantaneous voltagelevel of the input voltage V_(IN). In the controller 41 shown in FIG. 30the components corresponding to the components of the controller 4 shownin FIG. 14 have the same reference character to which a subscript index“1” has been added. With regard to the operation of the controller 4 ₁reference to the description of FIG. 14 is made. The multiplier 414 ₁shown in FIG. 30 may be omitted when the input voltage V_(IN) of themulti-cell converter 10 is a direct voltage. In this case, the inputsignal B of the divider corresponds to V_(IN) _(—) _(MAX).

As can be seen in the converter cell 1 ₁ shown in FIG. 29, the switchcontroller (19 in cell 1 ₁) of each converter cell 1 ₁-1 _(N1) receivesthe modulation index (m_(i) in cell 1 ₁) from the correspondingcontroller (41 in cell 1 ₁) and controls the switches (17 _(H)-18 _(L)in cell 1 ₁) in the converter cell based on the modulation index m₁. Theindividual main controllers 4 ₁ can be implemented in the convertercells 1 ₁-1 _(N). In case of a digital implementation of the maincontroller 4 ₁ and the switch controller 19 ₁, of one converter cell 1₁, the main controller 4 ₁ and the switch controller 19 may beimplemented in one signal processor.

FIG. 31 shows one embodiment of the second power converter 20 with an OPtopology, that is, a topology in which the cell outputs of theindividual converter cells 2 ₁-2 _(N3) are connected in parallel at theoutput OUT1, OUT2. In FIG. 31 only one converter cell, namely convertercell 2 ₁ is shown in detail. The other converter cells 2 ₂-2 _(N3) canbe implemented equivalently.

The converter cell 1 ₁ is implemented with a flyback converter topology.That is, the converter cell 2 ₁ includes a series circuit with anelectronic switch 202 and a primary winding 201 _(P) of a transformer201, wherein this series circuit is connected in parallel with the DClink capacitor 11 ₁ so as to receive the DC link voltage V2 ₁. Asecondary winding 201 _(S) is inductively coupled with the primarywinding 201 _(P). A rectifier circuit 203 is coupled to the secondarywinding 201 _(S) and provides the cell output current I2 ₁ to the celloutput and the output OUT1, respectively. A PWM (Pulse Width Modulation)controller 204 receives an output current signal I2 _(—) _(M) and theoutput current reference signal I2 _(—) _(REF) . The output currentsignal I2 ₁ _(—) _(M) represents the instantaneous current level (actualvalue) of the output current I2. The output current reference signal I2₁ _(—) _(REF) represents a desired current level of the output currentI2 ₁. This output current reference signal I2 ₁ _(—) _(REF) may varyover time as the power consumption of the load may change. In thistopology, the transformer 219 provides for a galvanic isolation betweenthe cell input and the cell output.

The PWM controller 204 is configured to generate a PWM drive signal S202which drives the electronic switch 202. Based on the output currentreference signal I2 ₁ _(—) _(REF) and on the output current signal I2 ₁_(—) _(M) the PWM controller 201 controls a duty cycle of the PWM drivesignal S202 such that a current level of the output current I2 ₁ has acurrent level which at least approximately equals the current leveldefined by the reference signal I2 ₁ _(—) _(REF). The switchingfrequency of the PWM drive signal S202 may be in the same range as theswitching frequency in the converter cells 1 ₁-1 _(N1) explained above,that is, between 18 kHZ and several 100 kHz. The duty cycle is a ratiobetween the on-time of the electronic switch 202 in one drive cycle andthe duration of the drive cycle. The on-time of the electronic switch202 is the time for which the electronic switch 202 is switched on inone drive cycle. The duration of one drive cycle of the electronicswitch 202 is the reciprocal of the switching frequency.

The electronic switch 202, like the other electronic switches explainedabove and below, may be implemented as a conventional electronic switchsuch as, for example, a MOSFET (Metal Oxide Semiconductor Field-EffectTransistor), an IGBT (Insulated Gate Bipolar Transistor), a JFET(Junction Field-Effect Transistor), a BJT (Bipolar Junction Transistor),a HEMT (High Electron Mobility Transistor), in particular a GaN HEMT, orthe like.

FIG. 32A shows another type of converter cell which may be used in themulti-cell converter 20 shown in FIG. 31. The converter cell 2 _(i)(wherein i denotes any of the order numbers 1 to N3) shown in FIG. 32 isimplemented with a dual active bridge (DAB) topology. Such topology isdisclosed in FIGS. 2a and 2b of Everts, J.; Krismer, F.; Van den Keybus,J.; Driesen, J.; Kolar, J. W., “Comparative evaluation ofsoft-switching, bidirectional, isolated AC/DC converter topologies,”Applied Power Electronics Conference and Exposition (APEC), 2012Twenty-Seventh Annual IEEE, pp. 1067-1074, 5-9 Feb. 2012, which isdisclosed herein by reference in its entirety. FIG. 32 shows oneembodiment of a converter cell 2 _(i) implemented with a “fullbridge-full bridge DAB topology” as disclosed in Everts et al.

Referring to FIG. 32A, the converter cell 2 _(i) includes a first (full)bridge circuit with two half-bridges each including a high-side switch211, 213 and a low-side switch 212, 214. The half-bridges of the firstbridge circuit are connected between the cell input nodes for receivingthe respective DC link voltage V2 _(i). A series circuit with aninductive storage element 221 and a primary winding 219 _(P) of atransformer 219 is connected between output nodes of the two halfbridges 211, 212 and 213, 214, respectively. An output node of onehalf-bridge is a circuit node common to the high-side switch 211, 213and the low-side switch 212, 214 of the half-bridge. The transformer 219provides for a galvanic isolation between the cell input and the celloutput, wherein the cell output is connected to the output OUT1, OUT2 ofthe power converter circuit. The transformer 219 includes a secondarywinding 219 _(S) which is inductively coupled with the primary winding219 _(P). A further inductive storage 220 element drawn in parallel withthe primary winding 219 _(P) in FIG. 32A represents the magnetizinginductance of the transformer 219.

A second bridge circuit with two half bridges each including a high-sideswitch 215, 217 and a low-side switch 216, 218 is coupled between thesecondary winding 219 _(S) and cell output nodes of the cell output.Each of these half-bridges 215, 216, and 217, 218, respectively,includes an input, which is a circuit node common to the high-sideswitch 215, 217 and the low-side switch 216, 218 of the respectivehalf-bridge. The input of a first half-bridge 215, 216 of the secondbridge circuit is connected to a first node of the secondary winding 219_(S), and the input of a second half-bridge 217, 218 of the secondbridge circuit is connected to a second node of the secondary winding219 _(S). The half-bridges of the second bridge circuit are eachconnected between the cell output nodes.

The switches 211-214, 215-218 of the first and second bridge circuitsshown in FIG. 8 may each be implemented to include a rectifier element(freewheeling element), such as a diode, connected in parallel with theswitch. These switches can be implemented as known electronic switches,such as MOSFETs (Metal-Oxide Field-Effect Transistors), IGBTs (InsulatedGate Bipolar Transistors), JFETs (Junction Field-Effect Transistors),HEMTs (HighElectron-Mobility Transistors), or the like. When theswitches 211-214, 215-218 are each implemented as a MOSFET, an internalbody diode of the MOSFETs can be used as rectifier element, so that noadditional rectifier element is required.

A control circuit 222 controls operation of the two bridge circuits. Forthis, each of the switches 211-214, 215-218 receives an individual drivesignal from the control circuit 24. These drive signals are referred toas S211-S214 and S215-S218 in FIG. 8. The control circuit receives theoutput current signal I2 _(i) _(—) _(M) and the output current referencesignal I2 _(i) _(—) _(REF) and is configured to drive the switches211-214, 215-218 such that the current level of the output currentI_(OUT) substantially corresponds to the current level defined by thereference signal I2 _(i) _(—) _(REF). There are several ways to drivethe switches 211-214, 215-218 to obtain this. According to oneembodiment, a duty cycle of the individual switches 211-214, 215-218 ismodulated in the region of 50%. For details on controlling the switches,reference is made to F. Krismer, J. W. Kolar “Closed form solution forminimum conduction loss modulation of DAB converters”, IEEE Transactionson Power Electronics, Vol. 27, Issue 1, 2012, which is incorporatedherein by reference in its entirety.

According to one embodiment, the control circuit 222 is configured tocontrol a timing of switching on and switching off the individualswitches 211-214 of the first bridge such that at least some of theswitches 211-214 are switched on and/or switched off when the voltageacross the respective switch is zero. This is known as zero voltageswitching (ZVS).

FIG. 32B shows another embodiment of one converter cell 2 _(i). In thisembodiment, the converter cell 2 _(i) is implemented with a buckconverter topology. The converter cell 2 _(i) includes a half-bridge 241with a high-side switch 241 _(H) and a low-side switch 241 _(L). Thehalf-bridge 241 is connected to the cell input such that that theassociated DC link voltage V2 _(i) drops across the series circuit withthe high-side switch 241 _(H) and the low-side switch 241 _(L). Aninductor is connected between a tap of the half-bridge 214 and one ofthe cell output nodes. The tap of the half-bridge is a circuit where thehigh-side switch 214 _(H) and the low-side switch 241 _(L) areconnected. A PWM controller 243 receives the output current signal I2_(i) _(—) _(M) which represents the output current I2 _(i) of theconverter cell and the output current reference signal which representsthe desired current level of the cell output current I2 _(i). The celloutput current I2 _(i) is the current through the inductor 242. Thecontroller 243 is configured to generate PWM drive signals S214 _(H),S241 _(L) for the high-side and the low-side switch 241 _(H), 241 _(L)switch such that the output current I2 _(i) has a current level whichsubstantially equals a current level represented by the output currentreference signal I2 _(i) _(—) _(REF).

In the buck topology shown in FIG. 32B, the current level of the outputcurrent I2 _(i) is controlled by controlling the duty cycle of thehigh-side switch 241 _(H). The low side switch acts as a freewheelingelement, which switches complementary to the high-side switch 241 _(H).

It should be noted that the topologies of the converter cells shown inFIGS. 31 and 32A-32B are only two of many possible examples of how theconverter cells 2 ₁-2 _(N3) may be implemented. The topologies shown inFIGS. 31 and 32A each include a transformer 201 and 219, respectively,which provides for a galvanic isolation between the cell inputs and thecell outputs. Thus, these transformer also provide for a galvanicisolation between the input IN1, IN2 and the output OUT1, OUT2 of apower converter circuit implemented with a multi-cell converter 20 asshown in FIG. 31. However, possible implementations of the convertercells 2 ₁-2 _(N3) are not restricted to topologies including atransformer, which may be referred to as isolated topologies. Instead,non-isolated topologies, which are topologies without a galvanicisolation between the cell input and the cell output, may be used aswell. One example of such non-isolated topology is the buck convertertopology shown in FIG. 32B.

Referring to FIG. 31, the multi-cell converter 20 includes a maincontroller 3 which generates the output current reference signals I2 ₁_(—) _(REF)-I2 _(N3) _(—) _(REF) received by the individual convertercells 2 ₁-2 _(N3). One embodiment of this main controller is shown inFIG. 33. The main controller 33 shown in FIG. 33 includes an outputvoltage controller 31 which receives an output voltage signal V_(OUT)_(—) _(M) and an output voltage reference signal V_(OUT) _(—) _(REF).The output voltage signal V_(OUT) _(—) _(M) represents an instantaneousvoltage level of the output voltage V_(OUT) and the output voltagereference signal V_(OUT) _(—) _(REF) represents the desired voltagelevel of the output voltage V_(OUT). Based on these signals V_(OUT) _(—)_(REF), V_(OUT) _(—) _(M), in particular based on a difference betweenthese signals V_(OUT) _(—) _(REF), V_(OUT) _(—) _(M), the output voltagecontroller 31 generates an output current signal I_(OUT) _(—) _(REF).The output current signal I_(OUT) _(—) _(REF) represents a desiredcurrent level of the output current I_(OUT). According to oneembodiment, the individual converter cells 2 ₁-2 _(N3) supply equalshares of the output current I_(OUT). In this case, the level of each ofthe output current reference signals I2 ₁ _(—) _(REF)-I2 _(N3) _(—)_(REF) received by the individual converter cells 2 ₁-2 _(N3) is I_(OUT)_(—) _(REF)/N3. In the embodiment shown in FIG. 33, a divider 31′calculates the output current reference signals I2 ₁ _(—) _(REF)-I2_(N3) _(—) _(REF) based on the output current reference signal I_(OUT)_(—) _(REF).

When implemented with a main controller 3 as shown in FIG. 33, themulti-cell converter 20 has a voltage source characteristic. Accordingto one embodiment, the second power converter 20 is configured toprovide the output power P_(OUT) such that the output voltage V_(OUT) issubstantially constant. As the power consumption of the load Z may vary,the second power converter 20, in this embodiment, is configured to varythe output current I_(OUT) in order to keep the output voltage V_(OUT)substantially constant, but to meet the power requirements by the loadZ. According to another embodiment, the second power converter 20 has acurrent source characteristic. That is, the second power converter isconfigured to control the output current I_(OUT). In this embodiment,the output voltage controller 31 may be omitted. In this case, theoutput current reference signal I_(OUT) _(—) _(REF) may be provided by acentral controller (not shown) similar to the central controller whichprovides the output voltage reference signal V_(OUT) _(—) _(REF).Basically, the second power converter may be configured to control oneof the output voltage V_(OUT) and the output current. This equivalentlyapplies to each of the second power converters 20 explained below, whichare configured to supply a direct current.

FIG. 34 shows one embodiment of the second power converter 20 with an OS(Output Serial) topology. In FIG. 34, converter cell 2 ₁ is shown indetail. The other converter cells 2 ₂-2 _(N3) may be implementedaccordingly. The OS topology shown in FIG. 34 is similar to the IStopology shown in FIG. 12. Like in the IS converter shown in FIG. 12,the OS converter shown in FIG. 33 has one inductor 24 which is connectedin series with the cell output of the individual converter cells 2 ₁-2_(N3). The series circuit with the cell outputs and the inductor 24 isconnected between the output nodes OUT1, OUT2.

In the embodiment shown in FIG. 34, the individual converter cells 2 ₁-2_(N3) are implemented with a full-bridge topology, which is explained indetail with reference to one converter cell 1 _(i) of the IS topology inFIG. 24 above. Referring to FIG. 34, the converter cell 2 ₁ includes afirst half-bridge 231 with a high-side switch 231 _(H) and a low-sideswitch 231 _(L), and a second half-bridge 232 with a high-side switch232 _(H) and a low-side switch 232 _(L). A controller 233 operates theseswitches 231 _(H)-232 _(L) by generating drive signals S231 _(H)-S232_(L) for these switches 231 _(H)-232 _(L) based on an modulation indexm_(i) received from a main controller 5. Unlike the converter cell 1_(i) shown in FIG. 24, a cell output of the converter cell 2 ₁ is formedby taps of the two half-bridges. The cell input, where the DC linkvoltage V2 ₁ is received, is formed by those circuit nodes where the twohalf-bridges 231, 232 are connected in parallel. The controller mayoperate the full-bridge in accordance with one of the modulation schemesexplained with reference to FIGS. 26A and 26B above.

The power converter 20 with the OS topology shown in FIG. 34 can beoperated to supply an output current I_(OUT) to a power grid connectedto the output nodes OUT1, OUT2. In this case, the output voltage V_(OUT)at the output OUT1, OUT2 is defined by the power grid. In other words,the power converter 20 receives the output voltage V_(OUT) at the outputand provides the output current I_(OUT) at the output. The instantaneouslevel of the output power is defined by the instantaneous level of theoutput voltage V_(OUT) and the instantaneous level of the outputcurrent. The output voltage may have a sinusoidal waveform, asschematically illustrated in FIG. 34. In this case, the second converter20 generates the output current I_(OUT) such that a waveform of theoutput current I_(OUT) is substantially in phase with the output voltageV_(OUT) (or that there is a predefined phase difference). Further, thesecond converter 20 may generate the amplitude of the output currentI_(OUT) such that the total DC link voltage has a predefined voltagelevel. A second power converter 20 configured to control the waveform ofthe output current I_(OUT) to substantially be equal to the waveform ofthe output voltage V_(OUT) will be referred to as a second powerconverter 20 with a PFC (Power Factor Correction) capability or,briefly, as a second PFC power converter 20.

In the embodiment shown in FIG. 34, the individual converter cells 2 ₁-2_(N3) share one inductor 34, which is connected in series with the celloutputs. According to another embodiment (not shown) each of theconverter cells 2 ₁-2 _(N3) includes an inductor connected between onecell output node and the tap of the first half-bridge 231. In each case,the individual converter cells 2 ₁-2 _(N3) at as buck converters. Thatis, the cell output voltage of each converter cell 2 ₁-2 _(N3) is lowerthan the DC link voltage V2 ₁-V2 _(N2) of the associated DC linkcapacitor 11 ₁-11 _(N3). The topology of the converter cell shown inFIG. 34 will also be referred to as full-bridge topology (or full bridgebuck topology) in the following.

The second converter 20 with the OS topology shown in FIG. 34 can beoperated to generate an AC voltage, such as a sine voltage, as theoutput voltage V_(OUT) from the DC link voltages V2 ₁-V2 _(N2). However,it can also be operated to generate a rectified sine voltage or a DCvoltage as the output voltage. In this case, the output voltage V_(OUT)is a rectified sine voltage or a DC voltage, the converter cell 2 ₁ maybe simplified by omitting the high-side switch 232 _(H) of the secondhalf-bridge 232 and by replacing the low-side switch 232 _(L) with aconductor. The converter cell 2 ₁ (and each of the other converter cells2 ₂-2 _(N3)) then only includes the first half-bridge 231, wherein thefirst half-bridges of the individual converter cells 2 ₁-2 _(N3) areconnected in series. Such modified topology of the converter cells 2 ₁-2_(N3) will be referred to as buck topology in the following.

It should be noted that the converter cells 2 ₁-2 _(N3) are notrestricted to be implemented with a full-bridge buck topology as shownin FIG. 34, or with a buck topology explained above. Other topologies,in particular modifications of the topology shown in FIG. 34 may be usedas well. One such modification is shown in FIG. 34. This modificationincludes an additional switch 234 connected between the DC linkcapacitor 11 ₁ and the full-bridge. Such modified topology is known asH5 topology. The further switch 234 may be switched on and offsynchronously with the one of the switches that is operated in the PWMmode in the respective modulation scheme. Another modification includesadditional switches (not shown) between the taps of the twohalf-bridges. Such modified topology is known as HERIC topology.

One embodiment of a main controller 5 which is configured to operate theconverter cells 2 ₁-2 _(N3) such that the second converter 20 controlsthe total DC link voltage V2 _(TOT) and generates the output currentI_(OUT) such that it is substantially in phase with the output voltageV_(OUT) is shown in FIGS. 35 and 36. FIG. 35 shows a block diagram ofone embodiment of the main controller 5, and FIG. 36 shows oneembodiment of the main controller shown in FIG. 35 in greater detail.The main controller 5 shown in FIGS. 35 and 36 is very similar to themain controller 4 of the IS converter shown in FIGS. 13 and 14. Insteadof the input reference current controller 41 in the main controller 4,the main controller 5 shown in FIG. 35 includes an output referencecurrent controller 51 which receives an output voltage signal V_(OUT)_(—) _(M) which represents the instantaneous voltage level of the outputvoltage V_(OUT). The output reference current controller 51 furtherreceives DC link voltage signals V2 _(I) _(—) _(M)-V2 _(N2) _(—) _(M)which represent the individual DC link voltages V2 ₁-V2 _(N2), and atotal DC link voltage reference signal V2 _(TOT) _(—) _(REF). The totalDC link voltage reference signal V2 _(TOT) _(—) _(REF) represents adesired signal level of the total DC link voltage V2 _(TOT). Based onthese signals, the output reference current controller 51 generates anoutput reference signal I_(OUT) _(—) _(REF) which is received by amodulation index controller 52. Based on the output current referencesignal I_(OUT) _(—) _(REF), and based on an output current signalI_(OUT) _(—) _(M), the modulation index controller 52 generates amodulation index m. According to one embodiment, the modulation indicesm₁-m_(N3) received by the individual converter cells 2 ₁-2 _(N3) shownin FIG. 34 is equal to the modulation index m generated by themodulation index controller 52. The output current signal I_(OUT) _(—)_(M) represents an instantaneous current level of the output current.

FIG. 36 shows one embodiment of the output reference current controller51 and the modulation index controller 52. The design and the operationof the output reference current controller 51 is similar to the designand the operation of the input reference current controller 41 of themain controller 4 shown in FIG. 14, to which reference is made.Referring to FIG. 36, the output reference controller 51 includes anerror filter 511 which receives the DC link voltage signals V2 ₁ _(—)_(M)-V2 _(N3) _(—) _(M) and the total DC link voltage reference signalV2 _(TOT) _(—) _(REF) and calculates an error signal V2 _(ERR) fromthese signals. The error filter 511 may have the same filtercharacteristic as explained with reference to the error filter 411 shownin FIG. 14. A multiplier 512 multiplies the error signal V2 _(ERR) withthe total DC link voltage reference signal V2 _(TOT) _(—) _(REF). Anoutput signal A of the multiplier 512 is received by a divider 513 whichdivides the output signal A of the multiplier 512 by a value which isdependent on the amplitude of the output voltage V_(OUT) (V_(OUT) _(—)_(MAX) ²/2 in this embodiment). An output signal C of the divider 513 isreceived by a further multiplier 514 which multiplies the divider outputsignal C with the output voltage signal V_(OUT) _(—) _(M). Like in themain controller 4 shown in FIG. 14, the divider 513 in the maincontroller 5 shown in FIG. 36 is optional. If the divider 513 isomitted, the further multiplier 514 receives the output signal A fromthe multiplier 512.

The second converter 20 shown in FIG. 34 is not restricted to supplypower to an AC grid. The second converter 20 may also supply power to aDC grid (a DC bus) which defines the output voltage V_(OUT). In thiscase, multiplying the divider output signal C or the multiplier outputsignal A with the output signal V_(OUT) _(—) _(M) may not be necessary.In this case, the input signal B of the divider is V_(OUT) _(—) _(MAX)instead of V_(OUT) _(—) _(MAX) ²/2.

The further multiplier 514 outputs the output current reference signalI_(OUT) _(—) _(REF). If the further multiplier 514 is omitted, eitherthe output signal C of the divider 513 or the output signal A of themultiplier 512 is the output current reference signal I_(OUT) _(—)_(REF).

Referring to FIG. 36 the modulation index controller 52 subtracts afiltered output current signal I_(OUT) _(—) _(F) from the output currentreference signal I_(OUT) _(—) _(REF) to generate an output current errorsignal I_(OUT) _(—) _(ERR). The filtered output current signal I_(OUT)_(—) _(F) is obtained by filtering the output current signal I_(OUT)_(—) _(M) by a first filter 522. The modulation index m is obtained byfiltering the output current error signal I_(OUT) _(—) _(ERR) by asecond filter. The first and second filter 522, 523 may be designed asexplained with reference to the filters 422, 423 shown in FIG. 14.

FIG. 37 shows one embodiment of the second converter 20 with an OPtopology. This converter 20 is based on the converter 20 shown in FIG.34 and is different from the converter shown in FIG. 34 in that theindividual converter cells 2 ₁-2 _(N3) have their cell outputs connectedin parallel at the output OUT1. In the embodiment shown in FIG. 37, eachconverter cell 2 ₁-2 _(N3) includes an inductor, which is shown in thefirst converter cell 2 ₁ where the inductor is labeled with referencecharacter 24 ₁. Further, each converter cell 2 ₁-2 _(N3) includes acontroller which generates the modulation index of the respectiveconverter cell. This is shown in the first converter cell 2 ₁ where thecontroller is labeled with reference character 5 ₁. The controller ineach converter cell, such as controller 5 ₁ in converter cell 2 ₁, maycorrespond to the controller 5 shown in FIGS. 35 and 36, with thedifference that the controller (in particular the error filter) of oneconverter cell only receives the DC link voltage signal of therespective converter cell and a DC link voltage reference signal of therespective converter cell instead of each of the DC link voltage signalsV2 ₁ _(—) _(M)-V2 _(N3) _(—) _(M) and the total DC link voltagereference signal V2 _(TOT) _(—) _(REF).

FIG. 38 shows another embodiment of a first power converter 10 with anIP topology. In this embodiment, the individual converter cells 1 ₁-1_(N1) are implemented with a dual-active bridge (DAB) topology, whereinonly the topology of the first converter cell 1 ₁ is shown in detail inFIG. 38. Referring to the converter cell 1 ₁, the cell topology is basedon the cell topology shown in FIG. 32. That is, the cell topologyincludes two full-bridges which each include two half bridges, whereineach half bridge includes a high-side switch 101, 103, 108, 110 and alow-side switch 102, 104, 109, 111. Like in the cell topology shown inFIG. 32, one full bridge is connected to the cell input (the full bridgewith switches 101-104 shown in FIG. 38), and one full bridge isconnected to the cell output (the full bridge with switches 108-111shown in FIG. 38). A primary winding 105 _(P) of a transformer 105 isconnected to taps of the first full bridges 101-I04, and a seriescircuit with a secondary winding 105 _(S) and a further inductor 107 isconnected to taps of the second full bridge 108-111. A further inductor106 drawn in parallel with the secondary winding 105 _(S) represents themagnetizing inductance of the transformer 105. A controller 112 controlsoperation of the individual switches of the full bridges by generatingdrive signals S101-S111 of the switches such that a cell input currentI0 ₁ has a current level defined by an input current reference signalI_(I) _(—) _(REF). For this, the controller 112 receives an inputcurrent signal I0 ₁ _(—) _(M) which represents an instantaneous currentlevel of the input current I0 ₁, and the input current reference signalI0 ₁ _(—) _(REF). A main controller 6 generates the input currentreference signals I0 ₁ _(—) _(REF)-I0 _(N1) _(—) _(REF) of theindividual converter cell 1 ₁-1 _(N1).

The converter cell 1 ₁ (like the other converter cells 1 ₂-1 _(N1))shown in FIG. 38 has one of a step-up and a step-down characteristic.That is, it may generate the DC link voltage V2 ₁ with a higher voltagelevel or with a lower voltage level than the input voltage V_(IN). Theconverter 10 shown in FIG. 38 is not restricted to be implemented withconverter cells having a DAB topology. Other topologies, such as theflyback topology, or the buck topology explained before may be used aswell.

FIG. 39 shows one embodiment of the main controller 6. In thisembodiment, the main controller 6 includes an input voltage controller61 which receives an input voltage signal V_(IN) _(—) _(M) and an inputvoltage reference signal V_(IN) _(—) _(REF) and is configured to controlthe voltage level of the input voltage V_(IN). The input voltagecontroller 61 generates an input current reference signal I_(IN) _(—)_(REF) based on these signals. In this embodiment, each of the inputcurrent reference signals I0 ₁ _(—) _(REF)-I0 _(N1) _(—) _(REF) receivedby the individual converter cells 1 ₁-1 _(N1) corresponds to the inputcurrent reference signal I_(IN) _(—) _(REF) generated by the inputvoltage controller 61. The input current signal I_(IN) _(—) _(REF)represents a desired current level of the input current I_(IN).According to one embodiment, the individual converter cells 1 ₁-1 _(N1)receive equal shares of the input current I_(IN). In this case, thelevel of each of the input current reference signals I1 ₁ _(—) _(REF)-I1_(N1) _(—) _(REF) received by the individual converter cells 1 ₁-1 _(N1)is I_(IN) _(—) _(REF)/N1. In the embodiment shown in FIG. 39, a divider61′ calculates the input current reference signals I1 ₁ _(—) _(REF)-I1_(N1) _(—) _(REF) based on the input current reference signal I_(IN)_(—) _(REF).

For example, the input voltage V_(IN) is controlled in thoseapplications where the input power is provided by a solar panel with aplurality of photovoltaic (PV) cells. The efficiency of a PV cell thatreceives solar power is dependent on the input voltage so that it maybecome necessary to vary the voltage at the PV panel as the solar powerreceived by the solar panel varies. An operation point at which a PVcell, at a given received solar power, has its maximum efficiency(supplies the maximum power) is referred to as maximum power point(MPP). The MPP can be found by varying the voltage at the PV cell andthe solar panel, respectively, and by measuring the power received fromthe solar panel. This is commonly known. According to one embodiment, anMPP tracker (not shown) which is configured to measure the powerreceived at the input IN1, IN2 provides the input voltage referencesignal V_(IN) _(—) _(REF) in order to operate the power source supplyingthe input voltage V_(IN) in the MPP.

According to another embodiment, a central controller (not shown)generates the input voltage reference signal V_(IN) _(—) _(REF).

According to another embodiment, the first power converter 10 isconfigured to control the input current I_(IN). In this embodiment, theinput voltage controller 61 may be omitted. In this case, the inputcurrent reference signal I_(IN) _(—) _(REF) may be provided by a centralcontroller (not shown) similar to the central controller which mayprovide the input voltage reference signal V_(IN) _(—) _(REF).

Several of a variety of different topologies of the first powerconverter 10 and the second power converter 20 are disclosed above. Inthe design of the power converter circuit, the type of the first powerconverter 10 and the type of the second power converter can be selecteddependent on the desired type of power conversion to be performed by thepower converter circuit. Some of a variety of combinations and theirpossible field of application are explained below. In the following, apower converter circuit configured to receive a periodic (alternating)input voltage (such as a sine voltage or a rectified sine voltage) andto supply a direct output voltage will be referred to as AC/DC powerconverter circuit, a power converter circuit configured to receive adirect input voltage and a periodic output voltage and configured tosupply an alternating output current will be referred to as AC/DC powerconverter circuit, and a power converter circuit configured to receive adirect input voltage and to provide a direct output voltage will bereferred to as DC/DC power converter circuit.

The design and ways of operation of a multi-cell power converter, suchas, for example, one of the IS, IP, OS, or OP multi-cell powerconverters explained herein before, offers several degrees of freedomwhich may be used in terms of increasing the efficiency of themulti-cell converter and of the power converter circuit in which it isemployed. These degrees of freedom include the number of converter cellsin one multi-cell converter, operation modes of the converter cells, thetype of connection between the converter cells, the voltage levels ofthe DC link voltages, the design of the converter cells, etc. Some ofthese degrees of freedom and how they may be used to increase theefficiency of a multi-cell converter are explained below.

In a multi-cell converter, such as one of the multi-cell convertersexplained herein before, each of the plurality of converter cells mayhave a maximum rated power. The maximum rated power defines the maximumpower the converter cell can convert. That is, the maximum input powerthe converter cell may receive, or the maximum output power theconverter cell may provide.

Referring to FIG. 40, the efficiency of the individual converter cellsmay vary dependent on a ratio between the instantaneous power and themaximum power. FIG. 40 schematically illustrates the efficiency of oneconverter cell based on this ratio. Referring to FIG. 40, the convertercell may have its maximum efficiency somewhere around 50% of the maximumpower, where the efficiency decreases towards lower output power levelsand towards higher power levels.

Referring to the explanation above, the individual converter cells canbe implemented as switched mode converter cells. That is, theseconverter cells are implemented as switched-mode power converters andeach include at least one semiconductor switch which is operated at aswitching frequency. For example, in an OP converter or an IP converter,the switched mode operation is used to control output currents I2 ₁-I2_(N3) and input currents I0 ₁-I0 _(N3) of the individual converter cells1 ₁-1 _(N1), 2 ₁-2 _(N3), respectively. The switching frequency may be18 kHz or higher. Switching on and off the at least one semiconductorswitch in a converter cell causes losses. These losses, which can bereferred to as switching losses, include a portion which issubstantially independent of the output power of the respectiveconverter cell. Those constant losses, which may result from losses indrivers, microcontrollers, or the like, are one reason that theefficiency of the converter cell significantly decreases as the outputpower decreases.

According to one embodiment, in order to efficiently operate themulti-cell power converter, that is, to efficiently convert the powerreceived by the multi-cell power converter, the individual convertercells in a multi-cell converter with an xP topology can be activated(operated in an active mode) or deactivated (operated in an inactivemode). A multi-cell converter with an xP topology is a multi-cellconverter with either an IP topology or a multi-cell converter witheither an OP topology. Deactivating of at least one converter cell in anxP topology may help to increase the efficiency of the other convertercells. This is explained for an OP topology with reference to FIGS.41A-44, and for an IP topology with reference to FIGS. 45A-49.

The individual converter cells in an xP topology can be referred to as“phases”. An operation mode in which at least one of these convertercells is inactive will be referred to as “phase shedding” mode in thefollowing. In the phase shedding mode active converter cells take overthe part of the inactive converter cell so that the total convertedpower only varies dependent on a power reference signal. The “powerreference signal” defines the power that is to be converted by themulti-cell converter.

For the purpose of explanation it is assumed that the power consumptionof the load Z is such that each of the converter cells 2 ₁-2 _(N) of asecond power converter 20 with an OP topology supplies an output powerwhich is significantly less than 50% of its maximum output power. If oneof the converter cells 2 ₁-2 _(N) is deactivated so that its outputpower becomes zero, the power levels of the other converter cells haveto increase in order to keep the power level of the output power P_(OUT)constant. However, the higher power levels of the other (active)converter cells may result in a higher efficiency of these activeconverter cells.

“Operating one converter cell in the inactive mode” means that, duringthe inactive mode, the cell input power received by the respectiveconverter cell and the cell output power provided by the respectiveconverter cell is substantially zero. Nevertheless, the DC linkcapacitor associated with the inactive converter cell may further becharged by the first power converter 10, as explained further below. Inthe active mode, the individual converter cells operate in aswitched-mode at a switching frequency fp as explained above. During theinactive mode, the DC link voltage of the respective converter cell mayincrease as the first converter 10 may further supply power to therespective DC link capacitor 11 ₁-11 _(N). This is independent of thespecific topology of the first power converter 10. The DC link voltageof the inactive converter cell may increase until the converter cell isagain activated and receives cell input power from the respective DClink capacitor. The DC link capacitors 11 ₁-11 _(N2) act as buffersbetween the first power converter 10 which receives input power from theinput IN1, IN2 and the second power converter 20 which provides theoutput power P_(OUT). The energy storage capability of these DC linkcapacitors 11 ₁-11 _(N) allows to cyclically operate the converter cells2 ₁-2 _(N3) in the inactive mode so as to increase the efficiency of thesecond power converter 20 when the output power P_(OUT) is low (atlow-load conditions).

FIG. 41A shows a first operation scenario of the second power converter20. In this scenario, one converter cell is in the inactive mode at onetime. In FIG. 41A only the activation states of the individual convertercells 2 ₁-2 _(N1) are shown. That is, the individual timing diagramsonly show whether the respective converter cell 2 ₁-2 _(N3) is active orinactive. The curves shown in FIG. 41A do not show the power level ofthe output power, and do not show the current level of the outputcurrent of the respective converter cells 2 ₁-2 _(N3). According toanother scenario, shown in FIG. 41B, two of the converter cells 2 ₁-2_(N) may be inactive at one time. In general, up to N3−1 converter cellscan be inactive at one time, leaving only one converter cell active atone time. Several different criteria may be used to decide which of theconverter cells belong to the group of N3−K active converter cells andto the group of K inactive converter cells at one time, and how long theindividual converter cells are active/inactive before a next decision istaken. This is explained in further detail below.

According to one embodiment, a number K of converter cells that areinactive at one time is set based on one of an output power referencesignals Pour REF and an output current reference signal I_(OUT) _(—)_(REF), respectively. This is illustrated in FIG. 42. The output powerreference signal P_(OUT) _(—) _(REF) defines a desired power level ofthe output power P_(OUT) to be provided by the second converter 20. Incase the output voltage V_(OUT) is substantially constant the outputcurrent reference signal I_(OUT) _(—) _(REF) is a measure for thedesired power level of the output power P_(OUT). Instead of the outputpower reference signal P_(OUT) _(—) _(REF) the instantaneous level ofthe output power P_(OUT), and instead of the output current referencesignal I_(OUT) _(—) _(REF) the instantaneous level of the output currentI_(OUT) may be used.

FIG. 42 illustrates K based on the output power reference signal P_(OUT)_(—) _(REF) and the output current reference signal I_(OUT) _(—) _(REF),respectively. In the embodiment shown in FIG. 42, none (K=0) of theconverter cells is inactive when the output power reference signalP_(OUT) _(—) _(REF) is above a first threshold P_(OUT) _(—) _(TH1) (ifthe output current reference signal I_(OUT) _(—) _(REF) is above a firstcurrent threshold I_(OUT) _(—) _(TH1)). One (K=1) converter cell isdeactivated (inactive) when the output power reference signal P_(OUT)_(—) _(REF) is between the first threshold P_(OUT) _(—) _(TH1) and asecond threshold P_(OUT) _(—) _(TH2) (when the output current referencesignal I_(OUT) _(—) _(REF) is between the first threshold I_(OUT) _(—)_(TH1) and a second current threshold I_(OUT) _(—) _(TH2)), two (K=2)converter cells are inactive when the output power reference signalP_(OUT) _(—) _(REF) is between the second power threshold P_(OUT) _(—)_(TH2) and a third power threshold P_(OUT) _(—) _(TH3) (when the outputcurrent reference signal I_(OUT) _(—) _(REF) is between the secondcurrent threshold I_(OUT) _(—) _(TH2) and a third current thresholdI_(OUT) _(—) _(TH3)), and so on. The difference between neighboringpower thresholds and current thresholds, respectively, can besubstantially the same or can be different.

FIG. 43 illustrates one embodiment of a method for setting the number Kof converter cells to be deactivated, and for identifying the cells thatare deactivated at one time. Referring to FIG. 43, the method includessetting the number M of cells to be operated in the inactive mode basedon the output current reference signal I_(OUT) _(—) _(REF) (1001). Theoutput current reference signal I_(OUT) _(—) _(REF) represents thedesired output current I_(OUT) of the second power converter 20.According to one embodiment, M is set based on the output currentreference signal I_(OUT) _(—) _(REF) in accordance with a curve as shownin FIG. 40.

Referring to FIG. 43, the method further includes identifying those Kconverter cells that currently have the lowest input voltage (DC linkvoltage) (1002). Such identification may include sorting the convertercells 2 ₁-2 _(N3) based on the voltage level of their DC link voltages11 ₁-11 _(N2) and selecting those K converter cells that have the lowestDC link voltage levels. These K identified converter cells are operatedin the inactive mode and the other converter cells are operated in theactive mode (1003). By operating the converter cells that have thelowest DC link voltages in the inactive mode, and, consequently,operating the other N3−K converter cells that have the highest DC linkvoltages in the active mode it can be prevented that the individual DClink voltages V2 ₁-V2 _(N2) (see, for example, FIG. 1) become toodifferent. Referring to FIG. 43, setting the number K of converter cells(1001), identifying those K cells with the lowest DC link voltage (1002)and operating the K identified cells in the inactive mode (1003) arerepeated. Repeating these process steps 1001-1003 may be time-based orevent based. Repeating these process steps 1001-1003 time-based mayinclude regularly repeating these steps 1001-1003. According to oneembodiment, a frequency at which the process steps 1001-1003 arerepeated is less than 0.1 times, or even less than 0.01 times theswitching frequency. According to one embodiment, the frequency at whichthe process steps 1001-1003 are repeated is 500 Hz or less.

Repeating these process steps 1001-1003 event-based may includerepeating these steps 1001-1003 every time a predefined event occurs.Examples of those events include, but are not restricted to, a decreaseof one DC link voltage V2 ₁-V2 _(N2) below a predefined first voltagethreshold, and the increase of one DC link voltage V2 ₁-V2 _(N2) above apredefined second voltage threshold, higher than the first threshold.According to another embodiment, the power drawn by the load at theoutput is measured and the process steps 1001-1003 are repeated when asignificant change of the power consumption of the load Z is detected.

According to one embodiment, the duration in which at least oneconverter cell is inactive is much longer than the period of one drivecycle of the converter cells in the active mode. According to oneembodiment, the duration in which the at least one converter cell isinactive is at least ten times the drive cycle period. Referring to theabove the drive cycle period Tp is the reciprocal of the switchingfrequency fp in the active mode.

Referring to the explanation above, the second power converter 20includes a main controller 3 which is configured to control operation ofthe individual converter cells 2 ₁-2 _(N3). FIG. 44 shows one embodimentof the controller 3 which is configured to operate at least on convertercell in the inactive mode. The controller 3 shown in FIG. 44 is based onthe controller 3 shown in FIG. 33 (to the specification of whichreference is made) and is different from the controller shown in FIG. 33in that it additionally includes a cell activation/deactivationcontroller 32. The cell activation/deactivation controller 32 receivesthe output current reference signal I_(OUT) _(—) _(REF) from the outputvoltage controller 31 which may be omitted when the output currentI_(OUT) is to be controlled). The operation of the cellactivation/deactivation controller 32 is based on the method explainedwith reference to FIG. 43. The activation/deactivation controller 32activates/deactivates the individual converter cells. That is, thecontroller 32 based on the output current reference signal I_(OUT) _(—)_(REF) sets the number K of converter cells that are to be deactivated,and based on the DC link voltages V2 ₁-V2 _(N2) of the individualconverter cells 2 ₁-2 _(N3) selects those cells that are to bedeactivated. For identifying those converter cells 2 ₁-2 _(N) that havethe lowest DC link voltage, the controller 32 receives DC link voltagesignals V2 ₁ _(—) _(M)-V2 _(N3) _(—) _(M) which represent the individualDC link voltages V2 ₁-V2 _(N3). These voltage signals V2 ₁ _(—) _(M)-V2_(N3) _(—) _(M) may be obtained from the individual DC link voltages V2₁-V2 _(N3) using conventional voltage measurement circuits (not shown inthe drawings).

Referring to FIG. 44, the cell activation/deactivation controllergenerates 32 current reference signals I2 ₁ _(—) _(REF)-I2 _(N3) _(—)_(REF). These reference signals I2 ₁ _(—) _(REF)-I2 _(N3) _(—) _(REF)represent desired current levels of the output currents I2 ₁-I2 _(N3) ofthe individual converter cells 2 ₁-2 _(N). The cellactivation/deactivation controller 32 generates the individual referencesignals I2 ₁ _(—) _(REF)-I2 _(N3) _(—) _(REF) such that their sumcorresponds to the output current reference signal. That is,

Σ_(i=1) ^(N3) I2_(i) _(—) _(REF) =I _(OUT) _(—) _(REF)  (21).

In this way, the converted power is only dependent on the output powerreference signal P_(OUT) _(—) _(REF) and the output current referencesignal I_(OUT) _(—) _(REF), respectively. Thus, operating the multi-cellconverter 20 in the phase shedding mode does not result in substantialvariations of the converted power. The converted power is one of theinput power the second converter 20 receives from the DC link capacitorsand the first power converter 10, respectively, and the output powersupplied to the load. The current reference signal of the at least oneconverter cell that is to be deactivated are set to zero by the cellactivation/deactivation controller 32 in order to set the output powerof the deactivated converter cell to zero.

According to one embodiment, the cell activation/deactivation controller32 is configured to generate the current reference signals of the activeconverter cells such that these reference signals are substantiallyequal, so that the active converter cells substantially provide the sameoutput current. However, this is only an example. According to a furtherembodiment, the cell activation/deactivation controller 32 is configuredto generate the individual reference signals I2 ₁ _(—) _(REF)-I2 _(N3)_(—) _(REF) of the active converter cells such that these referencesignals are different. According to one embodiment, the cellactivation/deactivation controller 32 generates the current referencesignal of an active converter cell such that the current referencesignal of one converter cell is dependent on the DC link voltage of therespective converter cell. The current reference signal may be generatedsuch that the current reference signal increases as the DC link voltageof the associated DC link capacitor increases. In this embodiment, thoseactive converter cells that have a higher DC link voltage supply ahigher output current than other active converter cells having a lowerDC link voltage.

According to another embodiment, the cell activation/deactivationcontroller 32 generates the reference signals I2 ₁ _(—) _(REF)-I2 _(N3)_(—) _(REF) of the active converter cells in accordance with efficiencycurves such that these converter cells operate in a high efficiencyrange. A high efficiency range is, for example a range in which theefficiency is at least 60% or at least 75% of a maximum efficiency.Referring to the explanation below, the individual converter cells mayhave their maximum efficiency or a high efficiency range at differentcurrents. In this case, operating the active cells at differentcurrents, additionally to phase shedding, may help to increase theoverall efficiency of the power converter 20.

It should be noted that the block diagrams of each of the controllersshown in the drawings, such as controller 3 shown in FIG. 44 and otherdrawings, and controllers 4, 5 and 6 shown in other drawings, merelyserve to illustrate the functionality of the respective controllerrather than its implementation. The individual function blocks may beimplemented using a conventional technology that is suitable toimplement a controller. Specifically, the function blocks of thecontroller 3 may be implemented as analog circuits, digital circuits, ormay be implemented using hardware and software, such as a microcontroller on which a specific software is running in order to implementthe functionality of the controller 3.

Operating of converter cells of a multi-cell power converter in anactive mode or an inactive mode, as explained with reference to FIGS.41A-44 above is not restricted to converter cells in a second powerconverter 20. Such activating or deactivating of converter cells inorder to efficiently operate a power converter circuit may also beapplied to the converter cells 1 ₁-1 _(N1) in a first power converter 10with an IP topology (in which cell inputs of the individual convertercells are connected in parallel). This is explained with reference toFIGS. 45A-49 below.

FIGS. 45A and 45B show timing diagrams that illustrate how convertercells 1 ₁-1 _(N1) of the first power converter 10 may operate in anactive mode or an inactive mode. In the embodiment shown in FIG. 45A,only one of the converter cells 1 ₁-1 _(N1) is deactivated at one time,in the embodiment shown in FIG. 35B, two of the converter cells 1 ₁-1_(N1) are deactivated at one time. In general, up to N1−1 of theconverter cells 1 ₁-1 _(N1) may deactivated at one time. Activating anddeactivating the converter cells 1 ₁-1 _(N1) of the first powerconverter circuit 10 is similar to activating and deactivating convertercells of the second power converter 20, with the difference that in thefirst power converter 10 the at least one converter cell is activated ordeactivated based on an input power reference signal P_(IN) _(—) _(REF).The input power reference signal P_(OUT) _(—) _(REF) defines a desiredpower level of the input power P_(IN) to be received by the firstconverter 10. In case the input voltage V_(IN) is substantially constantthe input current reference signal I_(IN) _(—) _(REF) is a measure forthe desired power level of the input power P_(IN). Instead of the inputpower reference signal P_(IN) _(—) _(REF) the instantaneous level of theinput power P_(IN), and instead of the output current reference signalI_(OUT) _(—) _(REF) the instantaneous level of the input current I_(IN)may be used.

Referring to FIG. 46, the number K of converter cells which aredeactivated at one time may increase as the input power reference signalP_(IN) _(—) _(REF), or the input current reference signal I_(IN) _(—)_(REF) decreases. The input current reference signal I_(IN) _(—) _(REF)represents a desired current level of the input current I_(IN).Referring to FIG. 46, one converter cell (K=1) may be deactivated if theinput power reference signal P_(IN) _(—) _(REF) falls below a firstthreshold P_(IN) _(—) _(TH) or the input current reference signal I_(IN)_(—) _(REF) falls below a first current threshold I_(IN) _(—) _(TH1),two converter cells (K=2) may be deactivated if the input powerreference signal P_(IN) _(—) _(REF) or the input current referencesignal I_(IN) _(—) _(REF) falls below a second threshold P_(IN) _(—)_(TH2), I_(IN) _(—) _(TH2), respectively, and three (K=3) convertercells are deactivated if the input power reference signal P_(IN) _(—)_(REF) and the input current reference signal I_(IN) _(—) _(REF) fallsbelow a third threshold P_(IN) _(—) _(TH3), I_(IN) _(—) _(TH3),respectively.

In the embodiments explained before, there are four converter cells 1₁-1 _(N1) (N1=4). However, this is only an example. The number N1 ofconverter cells that are connected in parallel are not restricted toN1=4. In general, two or more converter cells are connected in parallel.

FIG. 47 illustrates one embodiment of a method for deactivating at leastone of the converter cells 1 ₁-1 _(N1). The method includes setting thenumber K of converter cells that are to be operated in the inactive mode(1011). The method shown in FIG. 47 uses the input current referencesignal I_(IN) _(—) _(REF) to detect the desired input power and to setK. However, any other signal representing the instantaneous or thedesired input power may be used as well. Setting the number K may beperformed in accordance with the curve shown in FIG. 46. The methodfurther includes identifying those K cells that have the highest outputvoltages (DC link voltage) (1012), and operating those K identifiedcells in the inactive mode, and operating the other cells in the activemode (1013). The process steps 1011-1013 which include setting thenumber K, identifying those K cells that have the highest outputvoltage, and operating the K identified cells in the inactive mode canbe repeated periodically (time based), or event based. According to oneembodiment, an event based repetition of these process steps 1011-1013may include repeating the process steps 1004-1006 when the voltage levelof one DC link voltage V2 ₁-V2 _(N) of one of the plurality of convertercells 1 ₁-1 _(N) rises above a predefined first threshold level, or thevoltage level of one of the plurality of DC link voltages V2 ₁-V2 _(N)falls below a predefined second threshold level, lower than the firstthreshold.

FIG. 48 shows a block diagram of one embodiment of a main controller 6which is configured to activate or deactivate the individual convertercells 1 ₁-1 _(N1). This main controller 6 is based on the maincontroller 6 shown in FIG. 39 (to the specification of which referenceis made) and is different from this main controller 6 in that itadditionally includes a cell activation/deactivation controller 62. Thecell activation/deactivation controller 62 receives the input currentreference signal I_(IN) _(—) _(REF) from the input voltage controller 61(which may be omitted when the input current is to be controlled) andgenerates input current reference signal I0 ₁ _(—) _(REF), I0 ₂ _(—)_(REF), I0 ₃ _(—) _(REF), I0 _(N1) _(—) _(REF) for the individualconverter cells 1 ₁-1 _(N1). These input current reference signals I0 ₁_(—) _(REF)-I0 _(N1) _(—) _(REF) are received by the individualconverter cells 1 ₁-1 _(N), which are configured to control their inputcurrent I0 ₁-I0 _(N1) based on these reference signals I0 ₁ _(—)_(REF)-I0 _(N1) _(—) _(REF), as explained above.

The cell activation/deactivation controller 62 is configured to set thereference current of the at least one converter cell which is to bedeactivated to zero. According to one embodiment, the levels of theinput current reference signals of those converter cells that are to beactivated (operate in the active mode) are equal. According to anotherembodiment the cell activation/deactivation controller 52 is configuredto generate the input current reference signals of the activatedconverter cells with different current levels. For example, the cellactivation/deactivation controller 62 generates the signal levels of theactive converter cells based on the DC link voltages such that thereference signal decreases as the DC link voltage increase, in order tocharge the DC link capacitors of those converter cells 1 ₁-1 _(N1)slower that have a relatively high DC link voltage V2 ₁-V2 _(N2).

According to another embodiment, the activation/deactivation controller32 is configured to generate the current reference signals of the activeconverter cells based on efficiency curves of the active converter cellssuch that these converter cells operate in a high efficiency range. Ahigh efficiency range is, for example a range in which the efficiency isat least 60% or at least 75% of a maximum efficiency.

However, in each case the sum of the reference signals corresponds toinput current reference signal I_(IN) _(—) _(REF). That is,

Σ_(i=1) ^(N1) I0_(i) _(—) _(REF) =I _(IN) _(—) _(REF)  (22)

Thus, the input power is only dependent on the input power referencesignal P_(IN) _(—) _(REF) or the input current reference signal,respectively. Thus, operating the multi-cell converter 10 in the phaseshedding mode does not result in substantial variations of the convertedpower. The converted power is of the input power the second converter 10receives at the input, and the output power supplied to the DC linkcapacitors and the second converter, respectively.

Activating/deactivating converter cells 1 ₁-1 _(N) of the first powerconverter 10 based on an input current reference signal I_(IN) _(—)_(REF) may, in particular, be employed in a power converter circuitwhich receives a DC voltage as the input voltage V_(IN).

However, activating deactivating converter cells in a power converterwith an IP or OP topology is not restricted to power converters whichreceive or generate a DC voltage. Such activating or deactivating ofconverter cells may also be employed in a multi-cell converter of thetype shown in FIG. 29 which includes a plurality of converter cells 1₁-1 _(N1) that have the cell inputs connected in parallel, which has aPFC capability, and which receives a periodic input voltage V_(IN). Aswill be explained in further detail with reference to FIGS. 50 and 53below, the input power P_(IN) in such power converter periodicallyvaries with a frequency which is twice the frequency of the inputvoltage V_(IN). The input power is zero when the instantaneous level ofthe input voltage V_(IN) is zero and increases as the level of the inputvoltage increases until the input voltage V_(IN) reaches the maximum.After the input voltage has reached the maximum (or the minimum in thenegative halfwave), the input power decreases until the input voltageagain reaches zero. According to one embodiment, the converter cells 2₁-2 _(N1) are activated and deactivated based on at least one of thelevel of the input voltage V_(IN) and the input current I_(IN) such thatthe number of activated converter cells, within one halfwave, increasesas the input voltage V_(IN) and/or the input current increases anddecreases as the input voltage V_(IN) and/or the input currentdecreases. The order in which the converter cells are activated anddeactivated in one halfwave may change so that the DC link capacitors 11₁-11 _(N2) are equally charged. A controller (not shown in FIG. 29) mayactivate and deactivate the individual converter cells 1 ₁-1 _(N1) basedon at least one of the input voltage V_(IN) and the input currentI_(IN).

Equivalently, in a multi-cell converter of the type shown in FIG. 37which includes a plurality of converter cells 2 ₁-2 _(N3) that have thecell outputs connected in parallel, which has a PFC capability, andwhich receives a periodic output voltage V_(OUT) the converter cells 2₁-2 _(N3) may be activated and deactivated based on at least one of thelevel of the output voltage V_(OUT) and the output current I_(OUT) suchthat the number of activated converter cells, within one half-wave,increases as the output voltage V_(OUT) and/or the output currentI_(OUT) increases and decreases as the output voltage V_(OUT) and/or theoutput current I_(OUT) decreases. FIG. 49 shows a main controller 6according to another embodiment. In this embodiment, the main controller6 instead of the input voltage controller 61 includes a DC link voltagecontroller 60 which is configured to generate the input currentreference signal I_(IN) _(—) _(REF) based on a difference between thetotal DC link voltage V2 _(TOT) and desired DC link voltage.

Another way to increase the efficiency of a multi-cell power converterunder low load conditions is to intermittently operate the multi-cellpower converter such that an average converted power alternates.According to one embodiment, the multi-cell power converter is one of anIS, OS, IP, or OP power converter with a PFC functionality, such as oneof the IS, OS, IP, or OP power converters with PFC functionalityexplained hereinbefore. Operating such power converter in anintermittent operation mode is explained with reference to FIGS. 50-53.According to another embodiment, the multi-cell converter is one of anIS, OS, IP, or OP power converter configured to receive or provide adirect voltage. Operating such power converter in an intermittentoperation mode is explained with reference to FIGS. 54-59.

In general, in an IS, OS, IP or OP converter with PFC capability theconverted power periodically varies as the input voltage and the inputcurrent periodically vary. For example, if the input voltage V_(IN) inan IS or IP converter (or the output voltage V_(OUT) in an OS or OPconverter), is a sine voltage and the input current I_(IN) (the outputcurrent I_(OUT)) has a sine waveform, the converted power has a sinesquare waveform and a frequency which is twice the frequency of the sinevoltage. The converted power is the input power received at the inputIN1, IN2 in an IS or IP converter, and the output power P_(OUT) providedat the output OUT1, OUT2 in an OS or OP converter. In a normal mode (anon-intermittent mode) the average power level and the peak power levelof the converted power is only dependent on the power to be converted.This power to be converted may be defined by the input current referencesignal I_(IN) _(—) _(REF) and the output current reference signalI_(OUT) _(—) _(REF), respectively.

In the intermittent mode, the average power level and the peak powerlevel alternate. This is explained with reference to FIG. 50 whichschematically illustrates the waveform of an input voltage V_(IN) of anIS power converter, or the waveform of an output voltage V_(OUT) of anOS power converter. The voltage shown in FIG. 50 is sine voltage.However, the way of operation explained below applies to a rectifiedsine voltage equivalently. FIG. 50 further illustrates the input currentI_(IN), and the output current I_(OUT), respectively, and the inputpower P_(IN) and the output power P_(OUT), respectively.

In the embodiment shown in FIG. 50, the power converter converts poweronly during negative halfwaves of the input voltage V_(IN), or theoutput voltage V_(OUT), respectively. During these negative halfwaves,the waveform of the current I_(IN), I_(OUT) follows the waveform of thevoltage V_(IN), V_(OUT). That is, the current I_(IN)/I_(OUT) issubstantially in phase V_(IN)/V_(OUT), and a current level issubstantially proportional to a voltage level of the voltageV_(IN)/V_(OUT). The power P_(IN), P_(OUT) has a sine square waveformduring the negative half-cycles. FIG. 50 further illustrates the averagepower level P_(IN) _(—) _(AVG), P_(OUT) _(—) _(AVG) during the negativehalfwaves.

In the embodiment shown in FIG. 50, the multi-cell converter is operatedsuch that the current I_(IN), I_(OUT) and, therefore, the average powerlevel P_(IN) _(—) _(AVG), P_(OUT) _(—) _(AVG) is zero in the positivehalfwaves. However, this is only an example. In general, operating themulti-cell converter in the intermittent operation mode means operatingthe multi-cell converter such that the average power level alternatesbetween different levels wherein one of these levels is less than 80%,less than 50%, or even less than 30% of the other one of these levels.The “average power level” is the average power level in one half-period,that is, in a time period between two timely successive (sequential)zero crossings of the voltage V_(IN)/V_(OUT). This applies to a sinevoltage and a rectified sine voltage. In a sine voltage, zero crossingsare those times at which the voltage level is zero, that is at which thevoltage changes from a positive to a negative level, and vice versa. Ina rectified sine voltage, zero crossings are those times in which thevoltage becomes zero or close to zero before the voltage level againincreases.

In the embodiment shown in FIG. 50, the average power level P_(IN) _(—)_(AVG), P_(OUT) _(—) _(AVG) changes every halfwave so that the frequencyof changes of the average power level is twice the frequency of thevoltage V_(IN), V_(OUT). However, this is only an example. Instead ofreducing the average power only in every second halfwave (every positivehalfwave) there may two or more halfwaves in which the average power isreduced to the lower level before again having one halfwave in which theaverage power has the higher level. It is also possible, to have thehigher level for two or more subsequent halfwaves and then to change tothe lower level for one, two or more subsequent halfwave. In any case,in the intermittent mode, the average power level alternates betweendifferent levels. The average power levels between which the averagepower alternates may vary. That is, for example, the lower level maychange between a first time when the average power assumes the lowerlevel, and a second time after the first time.

FIG. 51 shows one embodiment of a main controller 4 of an IS powerconverter which has an intermittent operation functionality. The maincontroller 4 shown in FIG. 51 is based on the main controller 4 shown inFIG. 13 (to the description of which reference is made), and isdifferent from the main controller 4 shown in FIG. 13 in that itadditionally includes an intermittent operation controller between theinput reference current controller 41 and the modulation indexcontroller 42. The intermittent operation controller 43 receives theinput current reference signal I_(IN) _(—) _(REF) from the inputreference current controller 41 and is configured to provide a modifiedinput current reference signal I_(IN) _(—) _(REF)′ to the modulationindex controller 42. According to one embodiment, the intermittentoperation controller 43 is configured to generate the modified inputcurrent reference signal I_(IN) _(—) _(REF)′ such that the modifiedinput current reference signal I_(IN) _(—) _(REF)′ corresponds to theinput current reference signal I_(IN) _(—) _(REF) during certainhalfwaves of the voltage V_(IN)/V_(OUT), and that the modified inputcurrent reference signal I_(IN) _(—) _(REF)′ has a lower amplitude, suchas zero during certain halfwaves of the voltage V_(IN)/V_(OUT). In theembodiment shown in FIG. 50, the intermittent operation controller 43forwards the input current reference signal I_(IN) _(—) _(REF) to themodulation index controller 42 during the negative halfwaves of thevoltage V_(IN)/V_(OUT), and sets the modified input current referencesignal I_(IN) _(—) _(REF)′ to zero during the positive halfwaves of thevoltage V_(IN)/V_(OUT). During those time periods in which the modifiedinput current reference signal I_(IN) _(—) _(REF)′ is zero, themodulation index controller 42 generates the modulation index m suchthat the input current I_(IN) of the power converter is zero. Duringthis time period, the individual converter cells may still be operatedin a clocked fashion so that there may be time periods when themulti-cell power converter receives an input current. However, there arealso time periods in which the input current is negative (the multi-cellpower converter provides a current to the power source) so that theaverage input current is zero during those time periods in which themodified input current reference signal I_(IN) _(—) _(REF)′ is zero.

During those time periods in which the modified input current referencesignal I_(IN) _(—) _(REF)′ is zero, the multi-cell power converter maystill generate the cell input voltages V1 ₁-V1 _(N1) as explainedhereinbefore. In particular in those time periods in which the averageinput current should be zero, the main converter may operate only oneconverter cell in a clocked fashion in one drive cycle. This isexplained with reference to the timing diagram shown in FIG. 27.Referring to FIG. 27, the total cell input voltage V1 _(TOT) switchesbetween two voltage levels which are associated with a range of themodulation index m. In the embodiment shown in FIG. 27 the total cellinput voltage V1 _(TOT) switches between zero and V2 _(TOT)/N1 when themodulation index is between 0 and 0.25, switches between V2 _(TOT)/N1and V2 _(TOT)/N3 when the modulation index is between 0.25 and 0.5, andso on.

According to one embodiment, the multi-cell power converter is operatedsuch that only one converter cell is operated in a switched mode inorder to switch the total cell input voltage V1 _(TOT) between twodistinct voltage levels (such as, for example between V2 _(TOT)/N1 andV2 _(TOT)/N3) in the embodiment shown in FIG. 27 and to staticallyoperate the other converter cells. “To statically operate the otherconverter cells” means that one converter cells switches to theoff-state when the modulation index reaches a certain level and stays inthis off-state until the modulation index again falls below this certainlevel. For example, one converter cell may be switched off, when themodulation index reaches 0.25 in the embodiment shown in FIG. 27, inorder to provide one share of V2 _(TOT)/N1 to the total cell inputvoltage V1 _(TOT), and stays in the off-state until the modulation indexfalls below 0.25. This operation mode in which only one converter cellis operated in a switched and in which the other converter cells areoperated “statically” will be referred to as block mode in thefollowing. The number of converter cells which are statically operatedin the off-state increases as the voltage level of the input voltageincreases. That is, based on the voltage level of the input voltageV_(IN) the converter cells are operated in one of the PWM mode, theon-mode and the off-mode, wherein according to one embodiment, only oneconverter cell is operated in the PWM mode at on time.

In the embodiment shown in FIG. 50, the average power received/providedby the multi-cell power converter is zero during the negative halfwavesand is other than zero in the positive halfwaves. In this embodiment,the average power provided during the negative halfwaves is twice (twotimes) the average power that would be received/provide if themulti-cell converter would be operated continuously (notintermittently). However, as explained with reference to FIG. 40, theefficiency of the converter cells of the multi-cell power converter maydecrease as the power converted by the individual converter cellsdecreases. Intermittently operating the multi-cell converter, that is,operating the multi-cell power converter at a higher power duringcertain time periods (such as the negative halfwaves in the embodimentshown in FIG. 50), may increase the efficiency of the multi-cell powerconverter.

According to one embodiment, the intermittent operation controller 43calculates the average input power received during one halfwave and,based on this calculation, decides whether the multi-cell powerconverter is to be operated in the intermittent operation mode, or inthe normal mode. In the normal mode, the intermittent operationcontroller 43 passes the input current reference signal I_(IN) _(—)_(REF) to the modulation index controller 42. In the intermittentoperation, the ratio between those time periods in which the input poweris zero, and those time periods in which the input power is other thanzero (this ratio is 1:1 in the embodiment shown in FIG. 50) iscalculated based on the calculated power. Referring to FIG. 51 theintermittent operation controller may receive the input voltage signalV_(IN) _(—) _(M) and the input current signal I_(IN) _(—) _(M) in orderto calculate the average input power during one halfwave of the voltageV_(IN)/V_(OUT).

FIG. 52 shows one embodiment of a main controller 5 in an OS multi-cellpower converter having an intermittent operation functionality. Thismain controller 5 is based on the main controller 5 shown in FIG. 35 (tothe specification of which reference is made) and is different from themain controller 5 shown in FIG. 35 in that it includes an intermittentoperation controller 53 between the output reference current controller51 and the modulation index controller 52. This intermittent operationcontroller 53 receives the output current reference signal I_(OUT) _(—)_(REF) from the output reference current controller and provides amodified output current reference signal I_(OUT) _(—) _(REF)′ to themodulation index controller 52. The operation of the main controller 5shown in FIG. 52 may correspond to the operation of the main controller4 shown in FIG. 51, with the difference that the main controller 5 shownin FIG. 52 processes the output voltage signal V_(OUT/M) of the outputcurrent signal I_(OUT/M) instead of the input voltage signal V_(IN/M)and the input current signal I_(OUT/M). However, what has been explainedwith reference to the input voltage V_(IN) and the input current I_(IN)in an IS power converter applies to the output voltage V_(OUT) and theoutput current I_(OUT) in an OS power converter equivalently.

Operating a multi-cell power converter in an intermittent operation modeis not restricted to a multi-cell power converter with an IS topology oran OS topology. The intermittent operation mode explained with referenceto FIGS. 50 to 52 hereinbefore, may equivalently be used in an IP powerconverter of the type shown in FIG. 29, and an OP power converter of thetype shown in FIG. 37. FIG. 53 shows timing diagrams of the inputvoltage V_(IN) and the output voltage V_(OUT), respectively, of the cellinput current I0 _(i) and the cell output current I2 _(i), respectively,of one converter cell, and the input power P_(IN) and the output powerP_(OUT), respectively of the multi-cell converter with one of those IPand OP topologies. Each of these parallel connected converter cells maybe operated in an intermittent operation mode as explained before. Inthe intermittent mode, the average power converted by one converter cellalternates between a higher level and a lower level, wherein the lowerlevel may be less than 80%, less than 50%, or even less than 30% of thefirst level. Cell converters of these converter cells may correspond tothe main converters 4 and 5 shown in FIGS. 51 and 52, respectively, withthe difference that one controller instead of a total DC link voltagereference signal V2 _(TOT) _(—) _(REF) and each of the DC link voltagesignals V₁ _(—) _(M)-V1 _(N2) _(—) _(M) only processes a DC link voltagereference signal and the DC link voltage signal of the respectiveconverter cell.

According to one embodiment, in the intermittent operation mode of apower converter with parallel connected converter cells, the number ofconverter cells which operate at the lower level is the same in eachhalf-wave. If the first average power level of the individual convertercells is the same, and if the lower average power level of theindividual converter cells is the same, the total average power level(which is the sum of the average power levels of the individualconverter cells) is substantially the same in each halfwave. In thiscase, operating the individual converter cells in the intermittent modedoes not result in a varying average power level of the multi-cellconverter.

According to one embodiment, each converter cell in a power converterwith parallel connected converter cells is operated in an intermittentmode, and the converter cells are synchronized such that they changetheir average power level at the same time. In this case, the averagepower level of the multi-cell converter varies. This is shown in FIG. 53where the average converted power level P_(IN) _(—) _(AVG), P_(OUT) _(—)_(AVG) is drawn to vary. According to one embodiment, at least oneconverter cell is operated in the intermittent mode and at least oneconverter cell is operated in the normal mode. In this case, theconverted power P_(IN), P_(OUT) may have a waveform as shown in dashedlines in FIG. 53.

If, in one of the power converter circuits explained with reference toFIGS. 1 and 4-7 before, the first power converter 10 has an IS or IPtopology and is operated in an intermittent operation mode as explainedwith reference to FIGS. 50-53 before, the DC link capacitors 11 ₁-11_(N2) act as buffers which provide for a continuous power flow to thesecond power converter 20 and the load. In case the second powerconverter 20 is implemented with one of an OS topology and an OPtopology, the first power converter 10 may continuously draw power fromthe power source and charge the DC link capacitors 11 ₁-11 _(N2).

FIG. 54 shows one embodiment of operating a multi-cell power converterwith an IP topology or an OP topology, such as one of the topologiesexplained with reference to FIGS. 29 and 38 hereinbefore, in anintermittent mode. FIG. 54 shows timing diagrams of the activationstates of the individual converter cells. These converter cells areconverter cells 2 ₁-2 _(N3) in an OP power converter, and convertercells 1 ₁-1 _(N1) in an IP power converter. According to the embodimentshown in FIG. 54, operating the power converter in the intermittent modemay include activating only one converter cell at one time. In the timeperiod illustrated in FIG. 54, a first converter cell 2 ₁ and 1 ₁,respectively, and a second converter cell 2 ₁ and 2 ₂, respectively, isactivated. T_(OP) denotes an activation time, which is a time durationin which the respective converter cell is activated. These activationtimes are drawn to be equal in the embodiment shown in FIG. 54. However,this is only an example. These activation times may vary dependent ondifferent parameters. This is explained in further detail below. In theembodiment shown in FIG. 54, there is a time period between theactivation times of the converter cells 2 ₁, 2 ₂. Thus, the convertedpower P_(OUT) (P_(IN)) alternates. That is, there are times when thepower level of the converted power changes from a higher level to alower level, and times when the power level of the converted powerchanges from the lower level to the higher level. The higher level andthe lower level may vary. However, each time there is a change of thepower level from the higher level to the lower level the lower level isless than 80%, less than 50%, or even less than 30% of the higher level.

In the embodiment shown in FIG. 54, the lower level is zero. That is,there are times in which none of the converter cells is active. However,this is only an example. It is also possible to have one or moreconverter cells active while at least one other is operatedintermittently. In this case, the lower level is different from zero.

If, in one of the power converter circuits explained hereinbefore, thesecond power converter 20 is implemented as an OP converter with anintermittent operation functionality, an output capacitor 30 may providefor a continuous power flow to the load Z. This is schematicallyillustrated in FIG. 55. FIG. 55 shows one section of the power convertercircuit. Referring to FIG. 55, an output capacitor may be connectedbetween the output node OUT1, OUT2. This output capacitor 30 isintermittently supplied with power by the second power converter 20having an OP topology. However, by virtue of the charge storingcapability of the output capacitor 30 the load Z may continuously drawpower from the power converter circuit at the output OUT1, OUT2.

FIG. 56 shows one embodiment of a method for operating a multi-cellconverter with OP topology in an intermittent mode. Referring to FIG.56, the method includes evaluating the output current reference signalI_(OUT) _(—) _(REF) (1031). The output current reference signal isrepresentative of the desired output power of the multi-cell converter.Instead of the output current reference signal, another signalrepresenting the output power may be used as well. Evaluating the outputcurrent reference I_(OUT) _(—) _(REF) includes comparing the outputcurrent reference signal I_(OUT) _(—) _(REF) with an optimum outputcurrent signal I_(OUT) _(—) _(OPT) of one converter cell. This optimumoutput current signal represents an output power at which the convertercell either has its maximum efficiency or at which the efficiency of theconverter cell is not below a predefined efficiency level. Instead ofthe optimum output current signal another signal representing the outputpower at which the converter cell either has its maximum efficiency maybe used as well.

Referring to FIG. 56, if the output current reference signal I_(OUT)_(—) _(REF) is not below the optimum output current I_(OUT) _(—) _(OPT),the multi-cell converter is operated in a non-intermittent mode. Thismode is referred to as normal mode (1030) in FIG. 56. This normal modemay include phase shedding, so that in the normal operation mode, someof the converter cells can be inactive, as explained with reference toFIGS. 40 to 49 hereinbefore. However, in the normal operation mode atleast one converter cell is active at one time, so that there is no timeperiod in which each of the converter cells is inactive (deactivated).

Referring to FIG. 56, if the level of output current reference signalI_(OUT) _(—) _(REF) is below the level of the optimum output currentsignal I_(OUT) _(—) _(OPT) of one converter cell, the multi-cellconverter enters the intermittent mode in which the operation durationT_(OUP) is calculated (1032) as shown in FIG. 56. Then, the convertercell that has the highest input voltage is identified and the referencecurrent of the identified converter cell is set to I_(OUT) _(—) _(OPT)for the calculated time duration T_(OP), and the reference currents ofthe other converter cells are set to zero. According to one embodiment,the power converter first enters the phase shedding mode when the levelof the power reference signal (such as the output current referencesignal decreases), and finally enters the intermittent mode when thepower reference signal (such as the output current reference signaldecreases) further decreases.

According to one embodiment, the individual converter cells are designedto have substantially the same optimum output current I_(OUT) _(—)_(OPT). According to another embodiment, the individual converter cellsare designed to have different optimum output currents I_(OUT) _(—)_(OPT). In this embodiment, the intermittent operation mode may beginwhen the output reference signal I_(OUT) _(—) _(REF) falls below thelevel of the lowest optimum output current, then the converter cellhaving the highest input voltage is identified, and the operationduration is calculated based on the output current reference signalI_(OUT) _(—) _(REF) and the optimum output current I_(OUT) _(—) _(OPT)of the identified converter cell. The identified converter cell is thenoperated for the calculated duration at its optimum output current,while the other converter cells are operated at zero output current.

FIG. 57 shows one embodiment of a method for operating a multi-cellconverter with an IP topology in the intermittent mode. The method shownin FIG. 57 is based on the method shown in FIG. 56 to which thereference is made. The difference between the method explained withreference to FIG. 56 and the method illustrated in FIG. 57 is that inthe multi-cell converter with IP topology the input current referencesignal I_(IN) _(—) _(REF) is compared with an optimum input currentI_(IN) _(—) _(OPT) (see 1041 in FIG. 57) and that the operation durationT_(OP) is calculated based on the input current reference signal I_(IN)_(—) _(REF) and the optimum input current I_(IN) _(—) _(OPT). Everythingelse which has been explained with reference to the method illustratedin FIG. 56 applies to the method illustrated in FIG. 57 equivalently.

FIG. 58 shows one embodiment of a main controller 3 in a multi-cellconverter with OP topology. The main controller 3 is based on the maincontroller 3 shown in FIG. 33 and is different from this main controllershown in FIG. 33 in that an intermittent operation controller 33receives the output current reference signal I_(OUT) _(—) _(REF) fromthe output voltage controller 31 (which may be omitted when the outputcurrent is to be controlled) and generates the output current referencesignals I2 ₁ _(—) _(REF)-I2 _(N3) _(—) _(REF) in accordance with themethod explained with reference to FIG. 56. That is, the intermittentoperation controller 33 sets the signal level of the identifiedconverter cell to I_(OUT) _(—) _(OPT) for the calculated operationduration T_(OP).

FIG. 59 shows one embodiment of a main controller 6 in a multi-cellconverter with an IP topology and having an intermittent operationfunctionality. This main converter 6 shown in FIG. 59 is based on thecontroller 6 shown in FIG. 39 and is different from this controller 6shown in FIG. 39 in that it additionally includes an intermittentoperation controller 62 which receives the input current referencesignal I_(IN) _(—) _(REF) from the input voltage controller 61 (whichmay be omitted when the output current is to be controlled) andgenerates the input current reference signals I0 ₁ _(—) _(REF)-I0 _(N1)_(—) _(REF). In accordance with the method explained with reference toFIG. 57. That is, the intermittent operation controller 63 sets thesignal level of the identified converter cell to I_(IN) _(—) _(OPT) forcalculated operation duration T_(OP).

In each of the intermittent operation modes explained with reference toFIGS. 56 and 57, the output current reference signal I_(OUT) _(—) _(REF)and the input current reference signal I_(IN) _(—) _(REF), respectively,may be evaluated periodically. According to one embodiment, themulti-cell converter enters the intermittent mode when the powerreference signal falls below a first threshold (referred to as I_(OUT)_(—) _(OPT) and I_(IN) _(—) _(OPT) in the embodiments shown in FIGS. 56and 57) and leaves the intermittent mode when the power reference signalrises above a second threshold higher that the first threshold. Suchhysteresis prevents the multi-cell converter from frequently switchingbetween the intermittent and the non-intermittent mode when the powerreference signal has a level which is close to the first threshold.

FIG. 60 shows one embodiment of a power converter circuit in which thepower converter 10 has an IS topology and, additionally to the convertercells 1 ₁-1 _(N1), includes a filter cell 1 ₀. As explained before, theconverter cells 1 ₁-1 _(N1) are each configured to receive a cell inputpower at a cell input, and to provide a cell output power at a celloutput, to which the DC link capacitor 11 ₁-11 _(N2) is connectedthereto. The second power converter 20 is connected to the DC linkcapacitors 11 ₁-11 _(N2) of the first power converter 10. The secondpower converter 20 may be implemented with one of the second powerconverter topologies explained hereinbefore.

The filter cell 1 ₀ includes a capacitor 11 ₀ (which is drawn outsidethe block representing the filter cell 1 ₀ in FIG. 60). Unlike the DClink capacitors 11 ₁-11 _(N2), the capacitor 11 ₀ of the filter cell 1 ₀is not connected to the second power converter 20. The filter cell 1 ₀can be operated in an input power mode, in which the filter cellreceives an input power at an terminal of the filter cell, and an outputpower mode, in which the filter cell provides an output power at thegate of the filter cell 1 ₀. The terminal of the filter cell includestwo nodes and is connected in series with the cell inputs of theconverter cells 1 ₁-1 _(N1). The series circuit with the cell inputs ofthe converter cells 1 ₁-1 _(N1) and the terminal of the filter cell 1 ₀is connected to the input IN1, IN2 of the power converter circuit.

The filter cell 1 ₀ can be implemented with the same topology as theconverter cells 1 ₁-1 _(N1). One embodiment of the filter cell 1 ₀ isshown in FIG. 61. In the embodiment shown in FIG. 61, the filter cell 1₀ is implemented with a full-bridge topology, which is explained withreference to FIG. 24 above. In the filter cell 1 ₀ shown in FIG. 61, theindividual components have the same reference characters as thecorresponding components in the converter cell 1 shown in FIG. 24, wherea subscript index “0” has been added to the reference characters in thefilter cell 1 ₀ shown in FIG. 61. The operation of the filter cell 1 ₀corresponds to the operation of the converter cell 1 _(i). That is, acontroller 19 ₀ of the filter cell 1 ₀ receives a modulation index m₀and controls operation of the low-side switch 17 _(0L) and the high-sideswitch 17 _(0H) of the first half bridge 17 ₀ and of the low-side switch18 _(0L) and the high-side switch 18 _(0H) of the second half bridge 18₀ based on the modulation index m₀ in accordance with one of themodulation schemes explained with reference to FIGS. 26A and 26B.

Implementing the filter cell 1 ₀ with a full-bridge topology is only anexample. The filter cell 1 ₀ could also be implemented with only onehalf-bridge (as explained with reference to FIG. 12) when the inputvoltage V_(IN) is a rectified sine voltage or a direct voltage.

The operation of a first power converter 10 shown in FIG. 60 iscontrolled by a main controller 4. One embodiment of this maincontroller is shown in FIG. 62. This main controller 4 is based on themain controller 4 shown in FIG. 13 (to the specification of whichreference is made, and is different from the main controller 4 shown inFIG. 13 in that it additionally includes a converter and filter cellcontroller 44 which receives the modulation index from the modulationindex controller 42. The converter and filter cell controller 44provides a modulation index m₀ to the filter cell 1 ₀ and modulationindices m₁-m_(N1) to the converter cells 1 ₁-1 _(N1).

FIG. 63 illustrates one way of operation of the first power converter 10shown in FIG. 60 during one period of a sinusoidal input voltage V_(IN).In FIG. 63 V1 _(TOT) denotes the total cell input voltage and V10denotes the average cell input voltage of the filter cell, that is, thecell input voltage averaged over one or more drive cycles. In the firstpower converter 10 shown in FIG. 60, only the filter cell 1 ₀ isoperated in a switched-mode at the switching frequency such as, forexample 20 kHz or higher. The converter cells may be operated in theblock mode. That is, these converter cells may switch at a frequencywhich is twice the frequency of the input voltage V_(IN), so that eachconverter cell is switched to the off-state and the on-state only onceduring one halfwave of the input voltage V_(IN). That is, based in aninstantaneous voltage level of the input voltage V_(IN) the convertercells are operated in one of two operation modes, the on-mode or theoff-mode. However, it is also possible to switch the converter cells atthe switching frequency of the converter cell 1 ₀.

FIG. 64 shows one embodiment of a method for calculating the modulationindex m₀ of the filter cell, and the modulation indices of the convertercells. For the purpose of explanation it is assumed that the DC linkvoltages of the converter cells are substantially equal, that is, equalto V2 _(TOT)/N1. Referring to FIG. 64, the method includes calculating anumber F of converter cells that are to be operated in the off-state(1051). The total cell input voltage V1 _(TOT) provided by those Fconverter cells is F. V2 _(TOT)/N1. Calculating this number F includesdetermining the number F of converter cells to be operated in theoff-state by calculating

F=Round[m·N1]  (23),

that is, by the product of the modulation index m and the number N1 ofconverter cells, and by rounding the result. The modulation index m₀ ofthe filter cell 1 ₀ is then calculated (1052) based on the voltage V2 ₀across the capacitor 11 ₀ and the desired average voltage V1 ₀ at thegate of the filter cell 1 ₀ during one drive cycle by m₀=V1 ₀/V2 ₀,where V2 ₀ is the voltage across the capacitor 11 ₀ and V1 ₀ is thedesired voltage at the gate of the filter cell. The desired voltage V1 ₀at the terminal of the filter cell corresponds to the

$\begin{matrix}{{{V\; 1_{0}} = {{{m \cdot V}\; 2_{TOT}} - \frac{{F \cdot V}\; 2_{TOT}}{N\; 1}}},} & (24)\end{matrix}$

where m is the modulation index calculated by the modulation indexcontroller, and V2 _(TOT) is the total DC link voltage.

The converter and filter cell controller 44 then operates (1053) thefilter cell at the calculated modulation index m₀, operates F convertercells in the off-state (modulation index m_(i)=1), and operates N1-Fconverter cells in the on-state (modulation index m_(i)=0). Thisdetermining 1051, calculating 1052 and operating 1053 may be repeatedcyclically. According to one embodiment these steps are repeatedregularly. According to one embodiment, a frequency at which these stepsare repeated is less than 0.1 times, or even less than 0.01 times theswitching frequency in the filter cell 1 ₀.

Referring to the explanation above, the converter may be operated in theblock mode so that they are switched to the off-state and back to theon-state only once in each halfwave. In a multi-cell converterimplemented with a filter cell in which the converter cells are operatedin the block mode the filter cell may be optimized in terms of lowswitching losses, while the converter cells may be implemented in termsof low conduction losses.

The method illustrated in FIG. 64 applies to the positive half-wave ofthe input voltage. During the negative halfwave the method is differentfrom the method shown in FIG. 64 in that F is calculated based on theabsolute value of the modulation index (which is negative during thenegative half-wave) and that the F converter cells are operated at amodulation index m_(i)=−1

During one halfwave of the input voltage V_(IN) the converter cells areeither operated at m_(i)=1 (or −1) or m_(i)=0. The sign of themodulation index m₀ of the filter cell 1 ₀ may vary. That is, themodulation index m₀ can become positive and negative during onehalfwave. During the positive halfwave, when the modulation index m₀ ispositive, the filter cell 1 ₀ receives power from the input IN1, IN2.When the modulation index m₀ is negative, the filter cell 1 ₀ suppliespower to the series circuit with the converter cells 1 ₁-1 _(N1). Duringthe negative halfwave a positive modulation index m₀ indicates that thefilter cell 1 ₀ supplies power, and a negative modulation index m₀indicates that the filter cell 1 ₀ receives power. Thus, the filter cellreceives power (is in an input power mode) when the sign of themodulation index m₀ equals the sign of the overall modulation index m,and supplies power (is in the output power mode) when the signs aredifferent. Basically, the average power, the filter cell 1 ₀ receives inone half-period of the input voltage V_(IN) is zero so that the voltageV2 ₀ across the capacitor 11 ₀ swings around a certain voltage level,such as zero.

For example, during the positive halfwave, the modulation index ispositive when the sum of the cell output voltages of the F convertercells that are in the off-state is lower than the level of the inputvoltage, and is negative when the sum of the cell output voltages of theF converter cells that are in the off-state is higher than the level ofthe input voltage V_(IN). During the negative halfwave, the modulationindex is negative when the sum of the cell output voltages of the Fconverter cells that are in the off-state is lower than the (absolutevalue of the) level of the input voltage, and is positive when the sumof the cell output voltages of the F converter cells that are in theoff-state is higher than the (absolute value of the) level of the inputvoltage V_(IN).

FIG. 65 illustrates operation of the first power converter 10 shown inFIG. 60 during one drive cycle (having the duration Tp). In thisembodiment, two of the converter cells are in the off-state during thecomplete duration Tp of the drive cycle, and two of the converter cellsare in the on-state during the complete duration of the drive cycle Tp.The filter cell 1 ₀ is operated in a switched mode based on themodulation index m₀ (where duty cycle do is given by d₀=1−m₀).

Referring to the above, the converter cells may be operated in the blockmode. However, it is also possible to operate the multi-cell convertersuch that a group of converter cells that are in the off-state and,consequently, another group of converter cells that are in the on-state,may change from drive cycle to drive cycle. However, it is alsopossible, to operate the same converter cells in the off-state and thesame converter cells in the on-state until the modulation index m₀ ofthe filter cell 1 ₀ is calculated anew. In this way, the DC linkcapacitors are more equally charged.

FIG. 66 shows one embodiment of a second power converter 20 whichincludes a filter cell 2 ₀. Like the filter cell 1 ₀ shown in FIG. 60,the filter cell 1 ₀ includes an terminal. The terminal of the filtercell 2 ₀ is connected in series with cell outputs of the filter cells 2₁-2 _(N3). The series circuit with the terminal of the filter cell 2 ₀and the cell outputs 2 ₁-2 _(N3) of the converter cells is connected tothe output OUT1, OUT2. In order for the illustration shown in FIG. 66 tobe consistent with the illustration shown in FIG. 60, a capacitor of thefilter cell 2 ₀, like the capacitor shown in FIG. 60 is labeled with 11₀. V2 ₀ is the voltage across the capacitor of the filter cell 2 ₀. Theoperation of the filter cell 2 ₀ shown in FIG. 66 corresponds to theoperation of the filter cell shown in FIG. 60, with the difference thatthe filter cell 2 ₀ shown in FIG. 66 provides a voltage V3 ₀, which isadded to a total cell output voltage V3 _(TOT) of the converter cells.

FIG. 67 shows one embodiment of the filter cell 2 ₀. The full-bridgetopology of this filter cell 2 ₀ corresponds to the topology of theconverter cell 2 _(i) shown in FIG. 34. However, a topology with onlyone half-bridge may be used as well. Just to illustrate that the filtercell 2 ₀ is different from the converter cell a subscript index “0” hasbeen added to the reference characters of the individual components ofthe filter cell 2 ₀. The operation of the filter cell 2 ₀ corresponds tothe operation of the converter cell 2 _(i). That is, a controller 233 ₀of the filter cell 2 ₀ receives a modulation index m₀ and drives a firsthalf bridge 231 ₀ and a second half bridge 232 ₀ with a duty cyclecalculated based on the modulation index m₀.

FIG. 68 shows one embodiment of a main controller 5 in the second powerconverter 20 shown in FIG. 66. This main controller 5 is based on themain controller 5 shown in FIG. 35, and is different from the maincontroller 5 shown in FIG. 35 in that it additionally includes aconverter and filter cell controller 54 which receives the modulationindex m from the modulation index controller 52 and the provides themodulation index m₀ to the filter cell 2 ₀, and modulation indicesm₁-m_(N3) to the individual converter cells 2 ₁-2 _(N3). The converterand filter cell controller 54 shown in FIG. 68 operates like theconverter and filter cell controller 44 shown in FIG. 62, to whichreference is made. A difference between the converter and filter cellcontroller shown in FIG. 54 and the convergent filter cell controllershown in FIG. 62 is that the converter and filter cell controller shownin FIG. 55 generates the modulation indices m₀-m_(N3) based on the totalcell output voltage V3 _(TOT) instead of the total cell input voltage V1_(TOT).

FIG. 69 shows one embodiment of a method that may implemented in theconverter and filter cell controller 54 shown in FIG. 68. This methodincludes determining the number F of converter cells to be operated inthe off-state by

F=Round[m·N3]  (25),

that is, by calculating the product of the modulation index m and thenumber N3 of converter cells, and by rounding the result (1061). Thenthe modulation index m₀ is calculated similar to calculating themodulation index m₀ in the method shown in FIG. 64, with the differencethat the modulation index m₀ in the method shown in FIG. 69 iscalculated based on the desired cell output voltage V3 ₀ of the filtercell 2 ₀ (instead of the desired cell input voltage V1 ₀). The desiredcell output voltage V3 ₀ of the filter cell 2 ₀ is

$\begin{matrix}{{{V\; 3_{0}} = {{{m \cdot V}\; 2_{TOT}} - \frac{F\; {1 \cdot V}\; 2_{TOT}}{N\; 3}}},} & (26)\end{matrix}$

and the modulation index m₀=V3 ₀/V2 ₀. The converter and filter cellcontroller 54 then operates the filter cell 1 ₀ at the modulation indexm₀, F converter cells at a modulation index m_(i)=1, and N3-F convertercells at a modulation index m_(i)=0. The timing diagrams shown in FIG.63 apply to the second power converter 20 shown in FIG. 66 equivalently.The parameters of the second power converter 20 are indicated inbrackets in FIG. 63. The filter cell 2 ₀ supplies power (is in an outputpower mode) to the output OUT1, OUT2 when the modulation index m₀ has asign which equals the sign of the overall modulation index m, andreceives power (is in an input power mode) when the signs are different.The signs are equal, when the sum of the cell input voltages of thoseconverter cells which are in the off-state is lower than theinstantaneous level of the output voltage, and the signs are different,when the sum of these voltages is higher than the instantaneous level ofthe output voltage.

Another degree of freedom a multi-cell converter topology offers is thetype of connection between the individual converter cells. In theembodiments explained hereinbefore, the converter cells of onemulti-cell converter have their cell inputs connected in series (IStopology) or in parallel (IP topology), or have their outputs connectedin series (OS topology) or in parallel (OP topology). According to oneembodiment, the multi-cell converter includes at least two convertercells in which the type of connection between the two converter cellscan change between a parallel connection and a series connection. Thatis, these two converter cells are either series connected or parallelconnected. This is explained for two converter cells of a multi-cellfirst power converter 10 with reference to FIGS. 70-73, and for twoconverter cells of a multi-cell second power converter 20 with referenceto FIGS. 73-75.

In FIG. 70 reference character 1 _(k) and 1 _(k+1) denote two convertercells of the first power converter 10. 11 _(k), 11 _(k+1) denote thecorresponding DC link capacitors, and V2 _(k), V2 _(k+1) denote thecorresponding DC link voltages. Each of the converter cells 1 _(k), 1_(k+1) includes a cell input with a first cell input node, and a secondcell input node. A switch arrangement 7 is connected between the cellinputs of the converter cells 1 _(k), 1 _(k+1) and is configured toconnect the cell inputs either in series or in parallel. The switcharrangement 7 includes a first switch 71 connected between a first cellinput node of the converter cell 1 _(k+1) and a second cell input nodeof the converter cell 1 _(k). A second switch 72 is connected between afirst cell input node of the converter cell 1 _(k) and the first cellinput node of the converter cell 1 _(k+1). A third switch 73 isconnected between a second cell input node of the converter cell 1 _(k),and the second cell input node of the converter cell 1 _(k+1). Theconverter cells 1 _(k), 1 _(k+1) have their cell inputs connected inseries when the first switch 71 is switched on, and when the second andthird switch are switched off. In this case, the second cell input ofconverter cell 1 _(k) is connected to the first cell input of convertercell 1 _(k+1). The two converter cells 1 _(k), 1 _(k+1) have their cellinputs connected in parallel when the first switch 71 is switched off,and when each of the second and third switch 72, 73 is switched on. Inthis case, the first cell input node of converter cell 1 _(k) isconnected to first cell input node of converter cell 1 _(k+1), and thesecond cell input node of converter cell 1 _(k) is connected to thesecond cell input node of converter cell 1 _(k+1).

As indicated by the dotted lines shown in FIG. 70, the multi-cell powerconverter may include further converter cells, besides the convertercells 1 _(k), 1 _(k+1). The converter cells 1 _(k), 1 _(k+1) shown inFIG. 70 can be arranged in different ways in the multi-cell converter.According to one embodiment, the first cell input node of converter cell1 _(k) is connected to the first input node IN1 of the multi-cellconverter 10, and at least one further converter cell is connectedbetween the second cell input node of the converter cell 1 _(k+1) andthe second input node IN2 of the multi-cell converter 10. According toone embodiment, two or more converter cells are connected between theconverter cell 1 _(k+1) and the second input node IN2, wherein these twoor more converter cells have their cell inputs connected in seriesbetween the second cell input node of converter cell 1 _(k+1) and thesecond input node IN2. According to a further embodiment, the secondcell input node of the converter cell 1 _(k+1) is connected to thesecond input node IN2 of the multi-cell converter 10, and at least onefurther converter cell is connected between the first cell input node ofconverter cell 1 _(k) and the first input IN1. According to oneembodiment, two or more converter cells are connected between the firstinput node IN1 and the first cell input node of the converter cell 1_(k), wherein these two or more further converter cells have their cellinputs connected in series. According to another embodiment, two or moreconverter cells are connected between each input IN1, IN2 and theconverters cells 1 _(k), 1 _(k+1), respectively.

Further, each of the two converter cells 1 _(k), 1 _(k+1) includes aninductor (not shown in FIG. 70, but as explained with reference tovarious converter cell topologies above).

According to one embodiment, the converter cells 1 _(k), 1 _(k+1) shownin FIG. 70 (and the other converter cells which are not shown in FIG.70) have one of the converter topologies explained with reference toFIGS. 12 and 24 (boost topology or full-bridge topology).

FIG. 71 shows one way of operating the multi-cell power converter 10shown in FIG. 70. In this embodiment, the type of connection betweenconverter cell 1 _(k) and the converter cell 1 _(k+1) is dependent on aninstantaneous voltage level of the input voltage V_(IN). For example, ifthe voltage level of the input voltage V_(IN) is below a voltagethreshold V1, the two converter cells 1 _(k), 1 _(k+1) are connected inparallel. In FIG. 71 this is indicated by an on-level (high level) ofdrive signals S72, S73 of the second and third switches 72, 73, and anoff-level (low level) of a drive signal S71 of the first switch 71. Anon-level of one of the drive signal S71-S73 indicates that therespective switch is in the on-state, and an off-level indicates thatthe respective switch is in the off-state. When the voltage level of theinput voltage V_(IN) is above the voltage threshold V1, the convertercells 1 _(k), 1 _(k+1) are connected in series. In FIG. 71 this isindicated by an off-level of the drive signals S72, S73 of the secondand third switches 72, 73, and an on-level of the drive signal S71 ofthe first switch 71.

Each of the converter cells 1 _(k), 1 _(k+1) can be operated in one ofan on-state and an off-state. In the on-state, the cell input voltage V1_(k), V1 _(k+1) of each converter cell are substantially zero. When theconverter cells 1 _(k), 1 _(k+1) are connected in series, a total cellinput voltage V1 _(k) _(—) _(k+1) of the converter cells 1 _(k), 1_(k+1) is one of 0 (zero), V2 _(k), V2 _(k+1), and V2 _(k)+V2 _(k+1),dependent on which of the two converter cells 1 _(k), 1 _(k+1) is in theon-state or in the off-state. If the two converter cells 1 _(k), 1_(k+1) are connected in parallel, the total cell input voltage V1 _(k)_(—) _(k+1) is zero when both converter cells 1 _(k), 1 _(k+1) are inthe on-state. If both converter cells 1 _(k), 1 _(k+1) are in theoff-state (and the converter cells are implemented with a full bridgetopology), the total cell input voltage V1 _(k) _(—) _(k+1) is dependenton the voltage V2 _(k), V2 _(k+1) across the DC link capacitors 11 _(k),11 _(k+1). If these voltages are equal (V2 _(k)=V2 _(k+1)), the voltagelevel of the total cell input voltage V1 _(k) _(—) _(k+1) corresponds tothe voltage level of the DC link voltages V2 _(k), V2 _(k+1). If thesevoltages V2 _(k), V2 _(k+1) are not equal, there may be a chargebalancing such that electrical charge from the DC link capacitor whichhas the higher voltages transferred to the DC link capacitor which hasthe lower voltage until these voltages are balanced such that thevoltage levels of these two voltages V2 _(k), V2 _(k+1) are equal. Thevoltage level of the total cell input voltage V1 _(k) _(—) _(k+1) thenequals the voltage level of the balanced DC link voltages V2 _(k), V2_(k+1).

Referring to the above, the maximum level of the total cell inputvoltage V1 _(k) _(—) _(k+1) when the converter cells 1 _(k), 1 _(k+1)are connected in parallel is lower than the maximum level of the totalcell input voltage V1 _(k) _(—) _(k+)1 when the converter cells 1 _(k),1 _(k+1) are connected in series. When the input voltage V_(IN) is belowthe threshold V1, the lower cell input voltage (in connection with thecell input voltages of the other converter cells) may be sufficient totrack the voltage level of the input voltage V_(IN), while a higher cellinput voltage may be required when the voltage level of the inputvoltage V_(IN) is above the voltage threshold V1.

Connecting the two converter cells 1 _(k), 1 _(k+1) in parallel when theinput voltage V_(IN) is below the voltage threshold V1, may bebeneficial at a high level of the converted power. If, for example, theinput current I_(IN) is relatively high, even before the input voltageV_(IN) reaches the voltage threshold V1, so that the input power of oneconverter cell is higher than the power at which the converter cell mayhave its maximum efficiency, the two parallel connected converter cells1 _(k), 1 _(k+1) can share this input power so that each of theseconverter cells is operated at an efficiency which is higher than theefficiency of only one converter cell which converts the input power.For example, operating two parallel connected converter cell at a powerwhich is only 50% of the maximum power may be more efficient thanoperating only one converter cell at the maximum power.

FIG. 72 shows one embodiment of a main controller 4 which is configuredto control the converter cells in the multi-cell power converter 10shown in FIG. 70. This main controller 4 is based on the main controller4 shown in FIG. 13, and is different from the main controller shown inFIG. 13 in that it additionally includes a switch controller 45. Theswitch controller 45 receives the input voltage signal V_(IN) _(—) _(M)and is configured to operate the individual switches 71-73 of the switchcircuit 7 dependent on a voltage level of the input voltage V_(IN). Theswitch controller 45, which generates the drive signals S71-S73 of theswitches 71-73, may operate the switches 71-73 as shown in FIG. 71. Thatis, the switch controller 54 may operate the switches 71-73 such thatthe cell inputs of the converter cells 1 _(k), 1 _(k+1) are connected inparallel when a voltage level of the input voltage V_(IN) is below thethreshold V1, and connected in series when the voltage level of theinput voltage V_(IN) is above the threshold V1. The main controller 4shown in FIG. 72 is configured to operate the individual converter cellsof the multi-cell converter 10 with the same modulation index m.However, it is also possible to operate the individual converter cellswith different modulation indices.

According to one embodiment, the main controller 4 is configured toconnect the two converter cells 1 _(k), 1 _(k+1) in series and tooperate only one of the converter cells 1 _(k), 1 _(k+1) when the levelof the input voltage V_(IN) is below a second threshold lower than thefirst threshold V1, to connect both converter cells 1 _(k), 1 _(k+1) inparallel when the level of the input voltage V_(IN) is below the secondand the third threshold, and to connect the converter cells 1 _(k), 1_(k+1) in series again when the level of the input voltage V_(IN) isabove the first threshold. “Operating only one converter cell” of thetwo parallel cells is equivalent to controlling the input current of oneof the two cells to zero.

Although the multi-cell converter 10 shown in FIG. 70 is drawn toinclude only two converter cells which may be have their inputsconnected either in parallel or in series, the multi-cell converter 10is not restricted to having only two of those rearrangeable convertercells. “Rearrangeable” converter cells are converter cells that by aswitch circuit 7 may either have their cell inputs connected inparallel, or connected in series. According to one embodiment, themulti-cell converter 10 includes further rearrangeable converter cells.This may be obtained by providing a switch circuit of the type shown inFIG. 70 between two converter cells other than the converter cells 1_(k), 1 _(k+1) shown in FIG. 70. It is also possible, to provide aswitch arrangement of the type shown in FIG. 70 between one of theconverter cells 1 _(k), 1 _(k+1) and another converter cell (not shown).In this case, an arrangement of converter cells is obtained in which twoor three converter cells can be connected in parallel. According to oneembodiment, in a multi-cell converter 10 with N1 converter cells, thereare N1-1 switch arrangements, so that there is a switch arrangementbetween each pair of two neighboring converter cells. In thisembodiment, up to N1 converter cells can be connected in parallel.

In the embodiment shown in FIG. 70, there are two converter cells 1_(k), 1 _(k+1) that can be connected in parallel. According to anotherembodiment, each of the two converter cells is replaced with a seriescircuit (a string) of two or more converter cells. In this embodiment,the two strings are connected in parallel or in series based on thesignal level of the input voltage V_(IN), wherein the criteria forswitching the strings in series or in parallel can be the same asexplained with reference to the two converter cells 1 _(k), 1 _(k+1)above.

FIG. 73 shows one embodiment of second power converter 20 which includestwo rearrangeable converter cells 2 _(k), 2 _(k+1). Each of theseconverter cells 2 _(k), 2 _(k+1) includes a cell output with a firstcell output node and a second cell output node. A switch arrangement 8with a first switch 81, a second switch 82, and third switch 83 isconnected between the cell outputs of the two converter cells 2 _(k), 2_(k+1) such that a first switch 81 is connected between a second celloutput node of converter cell 2 _(k) and a first cell output node ofconverter cell 2 _(k+1), a second switch 82 is connected between thefirst cell output node of the converter cell 2 _(k) and the first celloutput node of the converter cell 2 _(k+1), and a third switch 83 isconnected between the second cell output node of the converter cells 2_(k) and the second cell output node of the converter cell 2 _(k+1). Thecell outputs of the converter cells 2 _(k), 2 _(k+1) are connected inseries when the first switch 81 is switched on and each of the secondswitch 82 and the third switch 83 is switched off, and the cell outputsare connected in parallel when the first switch 81 is switched off ineach of the second 82 and the third 83 is switched on.

In FIG. 73, 11 _(k), 11 _(k+1) denote the DC link capacitors connectedto the cell inputs of the converter cells 2 _(k), 2 _(k+1), and V3 _(k),V3 _(k+1) denote cell output voltages of the two converter cells 2 _(k),2 _(k+1). A power converter (not shown in FIG. 73) which supplies powerto the DC link capacitors 11 _(k), 11 _(k+1) may have any of theconverter topologies explained with reference to the first powerconverter hereinbefore. That is, the first power converter supplyingpower to the DC link capacitors 11 _(k), 11 _(k+1) shown in FIG. 73 notnecessarily is a first power converter with rearrangeable convertercells, although it may include rearrangeable converter cells.

According to one embodiment, the type of connection between the celloutputs of the two converter cells 1 _(k), 1 _(k+1) is dependent on avoltage level of the output voltage V_(OUT). According to oneembodiment, the output voltage V_(OUT) is defined by an external voltagesource such as, for example, a power grid. In this case, the multi-cellconverter 20 supplies the output power “against” the output voltageV_(OUT) defined by the external voltage source.

One way of operation of the multi-cell converter 20 shown in FIG. 73 isshown in FIG. 74. FIG. 74 shows the voltage level of the output voltageV_(OUT) during one halfwave of a sinusoidal output voltage. Referring toFIG. 74, the cell outputs of the converter cell 2 _(k), 2 _(k+1) may beconnected in parallel when the voltage level of the output voltageV_(OUT) is below a voltage threshold V2, and may be connected in series,if the voltage level of the output voltage V_(OUT) is above the voltagethreshold V2. A parallel connection of the converter cells 2 _(k), 2_(k+1) is indicated by on-levels (high levels) of drive signals S82, S83of the second switch 82 and the third switch 83 and an off-level (lowlevel) of a drive signal S81 of the first switch 81. A series connectionof the cell outputs is represented by off-levels of the drive signalsS82, S83 of the second switch 82 and the third switch 83 and an on-levelof drive signal S81 of the first switch 81.

In FIG. 73, V3 _(k) _(—) _(k+1) denotes a total cell output voltage ofthe two converter cells 2 _(k), 2 _(k+1). If the converter cells 2 _(k),2 _(k+1) are connected in series this total cell output voltage V3 _(k)_(—) _(k+1) is V2 _(k) (if 2 _(k) is in the off-state, and 2 _(k+1) inthe on-state), V2 _(k+1) (if 2 _(k) is in the on-state, and 2 _(k+1) isin the off-state), and V2 _(k)+V2 _(k+1) (if both, 2 _(k) and 2 _(k+1)are in the off-state). If the converter cells 2 _(k), 2 _(k+1) areconnected in parallel, the total cell output voltage V3 _(k) _(—) _(k+1)corresponds to a balanced voltage level. The balanced voltage level isthe voltage level obtained by charge balancing between the two DC linkcapacitors 11 _(k), 11 _(k+1) when both converter cells 2 _(k), 2 _(k+1)are in the off-state.

Like in the power converter 10 shown in FIG. 70, there is an inductor(not shown in FIG. 73) in each of the converter cells 2 _(k), 2 _(k+1).Further, other converter cells of the multi-cell converter 20 are notshown in FIG. 73. These converter cells may be connected between theconverter cell 2 _(k) and the output node OUT1, between the convertercell 2 _(k+1) and the second output node OUT2. Alternatively, one ormore converter cells are connected between each of the converters 2 _(k)and 2 _(k+1) and each of the output nodes OUT1, OUT2.

FIG. 75 shows one embodiment of a main controller 5 which is configuredto control operation of the second power converter 20 shown in FIG. 73.This main controller 5 shown in FIG. 75 is based on the main controller5 shown in FIG. 35 and is different from the main controller shown inFIG. 35 in that it additionally includes a switch controller 55 whichgenerates the drive signals S81, S82, S83 of the individual switches inthe switch arrangement 8 shown in FIG. 73. The switch controller 55 canbe configured to drive these switches 81-83 in accordance with theembodiment shown in FIG. 74 such that the switch arrangement 8 connectsthe cell outputs of the converter cells 2 _(k), 2 _(k+1) in series whena voltage level of the output voltage V_(OUT) is above the threshold V2,and to connect the cell outputs in parallel when the voltage level ofthe output voltage V2 is below the threshold.

Referring to FIGS. 71 and 74, the rearrangeable converter cells 1 _(k),1 _(k+1) and 2 _(k), 2 _(k+1), respectively, in the multi-cellconverters 10, 20, shown in FIGS. 70 and 73 are rearranged twice in eachhalfwave input voltage V_(IN) and the output voltage V_(OUT)respectively. In the embodiments shown in FIGS. 71 and 74, the type ofconnection changes from a parallel connection to a series connectionwhen the respective voltage rises above a threshold (V1, V2 in FIGS. 71and 74), and from the series connection back to a parallel connectionwhen the respective voltage falls below the threshold.

FIGS. 76A and 76B show one way of operation of a first power converter10 with an IP topology. FIG. 76A illustrates a power level of the inputpower P_(IN) (in case the input power is an AC power, P_(IN) denotes theaverage input power in one period of the input voltage V_(IN)), and theshares of the input power P_(IN) the individual converter cells 1 ₁-1_(N1) receive. Just for the purpose of explanation it is assumed thatthe power converter includes N1=3 converter cells, wherein P_(IN1),P_(IN2), P_(INN1) denote the input powers of the individual convertercells, and P_(IN1) _(—) _(REL)=P_(IN1)/P_(IN), P_(IN2) _(—)_(REL)=P_(IN2)/P_(IN), P_(INN1) _(—) _(REL)=P_(IN3)/P_(IN) denote theshares of the individual converter cells, wherein

P _(IN)=Σ_(i=1) ^(N1) P _(INi) _(—) _(REL)=100%  (27),

where P_(INi) _(—) _(REL) denote the input power shares of theindividual converter cells, and N1=3 in this embodiment.

Referring to FIG. 76A, the input power shares P_(IN1) _(—)_(REL)-P_(INN1) _(—) _(REL) of the individual converter cells 1 ₁-1_(N1) are dependent on the power level of the input power P_(IN) whichmay vary between a maximum level P_(IN) _(—) _(MAX) and a minimum levelP_(IN) _(—) _(MIN). In the embodiment shown in FIG. 76A, at the maximumlevel, P_(IN) _(—) _(MAX) converter cell 1 ₁ has the highest share,converter cell 1 _(N1) has the lowest share, and converter cell 1 ₂ hasa share lower than the one of converter cell 1 ₁ but higher than the oneof converter cell 1 _(N1). At the lowest level P_(IN) _(—) _(MIN) (whichis different from zero), converter cell 1 ₁ has the lowest share,converter cell 1 _(N1) has the highest share, and converter cell 1 ₂ hasa share lower than the one of converter cell 1 _(N1) but higher than theone of converter cell 1 ₁. In FIG. 76A, the dashed and dotted lineillustrates the input power share of one converter cell in a powerconverter in which the individual converter cells receive equal sharesof the input voltage independent of the level of the input power P_(IN).These shares are dependent on the number of converter cells. In a powerconverter with N1=3 converter cells, each converter cell receives 33.33%(=1/N1) of the input power P_(IN).

In the embodiment shown in FIG. 76A, the distribution of the input powershares is independent of the input power level if the input power levelis between the maximum level P_(IN) _(—) _(MAX) and a first level P_(IN)_(—) ₁, wherein, for example, P_(IN1) _(—) _(REL)=60%, P_(IN2) _(—)_(REL)=30%, P_(INN1) _(—) _(REL)=10%. If the input power level fallsbelow the first level P_(IN) the share of the converter 1 ₁ decreases asthe input power level decreases, while the share of the converter 1_(N1) increases. If the input power level falls below a second levelP_(IN) _(—) ₂ lower than the first level P_(IN) _(—) ₁ the share of theconverter 1 ₂ decreases as the input power level decreases, while theshare of the converter 1 _(N1) further increases. If the power level isbetween a third level P_(IN) _(—) ₃ lower than the second level P_(IN)and the minimum level P_(IN) _(—) _(MIN), the distribution of the inputpower shares is again independent of the input power level, wherein, forexample, P_(IN1) _(—) _(REL)=10%, P_(IN2) _(—) _(REL)=15%, P_(INN1) _(—)_(REL)=75%.

The distribution of the power shares at the individual input powerlevels shown in FIG. 76A is only an example. While in the embodimentshown in FIG. 76 each of the converter cells changes its input powershare as the input power level decreases, it is also possible to haveonly two converter cells which change their input power shares and toleave the input power share of the other converter cell(s) substantiallyconstant.

According to one embodiment, the individual converter cells 1 ₁-1 _(N1)are configured to receive a direct voltage as the input voltage V_(IN).In this case, the input powers of the individual converter cells can beadjusted by adjusting their respective input currents I0 ₁-I0 _(N1).FIG. 76B illustrates the distribution of the input currents I0 ₁-I0_(N1) dependent on the power level of the input power. Referring to FIG.76B, the input current I_(IN) decreases linearly as the power level ofthe input power P_(IN) decreases from the maximum level P_(IN) _(—)_(MAX) to the minimum level P_(IN) _(—) _(MIN). The individual inputcurrents I0 ₁-I0 _(N1), however, do not decrease linearly over thecomplete input power range. There may even be ranges where the inputcurrent of one converter cell is substantially constant or increase asthe power level decreases. For example, in the embodiment shown in FIG.76B, the input current I0 _(N1) of converter cell 1 _(N1) increases asthe input power level decreases between the second level and the thirdlevel P_(IN) _(—) ₂ and P_(IN) _(—) ₃. In general, the capability of thepower converter to unequally distribute the input power shares of theindividual converter cells may be used to keep the input power level(input current level) of at least one converter cell within a predefinedpower range (current range) in which the respective converter cell has ahigh efficiency, that is, for example an efficiency higher than 60% orhigher than 80% of the maximum efficiency.

In FIG. 76B, I0 ₁-I0 _(N1) denote the average input currents of theindividual converter cells. That is, the power converter may be operatedin a phase shedding mode or intermittent mode as the input power leveldecreases. In this case, there may be time periods in which theinstantaneous current levels of one or more of the input currents I0₁-I0 _(N1) become zero.

FIG. 77 shows one embodiment of a main controller 6 which is configuredto control the individual converter cells 1 ₁-1 _(N1) in the wayexplained with reference to one of FIGS. 76A and 76B. The maincontroller 6 shown in FIG. 77 is based on the main controller 6 shown inFIG. 39 and is different in that it includes a power share controller 64which is configured to generate the input current reference signals I0 ₁_(—) _(REF)-I0 _(N1) _(—) _(REF) of the individual converter cells basedon desired input power level. The power share controller may calculatethe desired input power level based on the input current referencesignal I_(IN) _(—) _(REF) (which may be calculated by the input voltagecontroller 61, received by a central controller, or by an MPP tracker)and the input voltage signal V_(IN) _(—) _(M). According to anotherembodiment, the power share controller 64 generates the input currentreference signals I0 ₁ _(—) _(REF)-I0 _(N1) _(—) _(REF) only based onthe input current reference signal I_(IN) _(—) _(REF).

The power share controller 64 is configured the generate the inputcurrent reference signals I0 ₁ _(—) _(REF)-I0 _(N1) _(—) _(REF) suchthat the (average) input currents I0 ₁-I0 _(N1) of the individualconverter cells are controlled as explained with reference to FIGS. 76Aand 76B before. The power share controller 64 may additionally have aphase shedding capability. That is, the power share converter mayalternately operate one or more of the converter cells in the active andthe inactive mode in order to control the input current of therespective converter cell.

The method explained with reference to one of FIGS. 76A and 76B in whichthe input power is unequally shared by the individual converter cells,is not restricted to be employed in a power converter with an IPtopology such as one of the first power converters with an IP topologyexplained before. Instead, this kind of operation may be used in amulti-cell converter with an OP topology, such as the multi-cellconverter shown in FIG. 31, as well. That is, a multi-cell converterwith an OP topology can be configured to vary the distribution of theoutput power shares of the individual converter cells 2 ₁-2 _(N3) basedon a power level of the output power P_(OUT). In FIGS. 76A and 76B theoutput power shares and output currents occurring in an OP converter areindicated in brackets. Here, P_(OUT1) _(—) _(REL)=P_(OUT1)/P_(OUT),P_(OUT2) _(—) _(REL)=P_(OUT2)/P_(OUT), P_(OUTN3) _(—)_(REL)=P_(OUTN3)/P_(OUT), denote the shares of the individual convertercells, wherein

P _(OUT)=Σ_(i=1) ^(N3) P _(OUTi) _(—) _(REL)=1.00%,  (28).

FIG. 78 shows one embodiment of a main controller 3 which is configuredto control the individual converter cells 2 ₁-2 _(N3) in the wayexplained with reference to one of FIGS. 76A and 76B. The maincontroller 3 shown in FIG. 78 is based on the main controller 3 shown inFIG. 33 and is different in that it includes a power share controller 34which is configured to generate the output current reference signals I2₁ _(—) _(REF)-I2 _(N3) _(—) _(REF) of the individual converter cellsbased on desired input power level. The power share controller 34 maycalculate the desired output power level based on the output currentreference signal I_(OUT) _(—) _(REF) (which may be calculated by theoutput voltage controller 31, or received by a central controller) andthe output voltage signal V_(OUT) _(—) _(M). According to anotherembodiment, the power share controller 34 generates the output currentreference signals I2 ₁ _(—) _(REF)-I2 _(N3) _(—) _(REF) only based onthe output current reference signal I_(OUT) _(—) _(REF).

The power share controller 34 is configured the generate the outputcurrent reference signals I2 ₁ _(—) _(REF)-I2 _(N3) _(—) _(REF) suchthat the (average) output currents I2 ₁-12 _(N3) of the individualconverter cells are controlled as explained with reference to FIGS. 76Aand 76B before. The power share controller 34 may additionally have aphase shedding capability. That is, the power share converter mayalternately operate one or more of the converter cells in the active andthe inactive mode in order to control the input current of therespective converter cell.

In an IP or OP multi-cell converter that operates in accordance with themethod illustrated in FIGS. 76A and 76B, the individual converter cells1 ₁-1 _(N1) (2 ₁-2 _(N3)) may be implemented differently in terms oflosses that may occur. Each type of converter cell explained aboveincludes at least one electronic switch. According to one embodiment,the individual converter cells 1 ₁-1 _(N1) (2 ₁-2 _(N3)) are designeddifferently in terms of conduction losses. According to one embodiment,this is obtained by designing the at least one electronic switch in atleast two of the converter cells 1 ₁-1 _(N1) (2 ₁-2 _(N3)) withdifferent on-resistances (R_(ON)). The on-resistance of an electronicswitch is the electrical resistance the electronic switch has in theon-state (switched on state). For example, the on-resistance of a MOSFETwhen used as an electronic switch is the electrical resistance between adrain node (D) and a source node (S) of the MOSFET in the on-state. AMOSFET can be designed to include a plurality of transistor cells thatare connected in parallel. In this case, the on-resistance issubstantially proportional to the number of transistor cells and,therefore, proportional to the area the MOSFET consumes on asemiconductor chip. In the design of a MOSFET the on-resistance can beadjusted by suitably selecting the number of parallel connectedtransistor cell. In a GaN-HEMT, as another example of an electronicswitch, the on-resistance can be adjusted by suitably selecting achannel width in the design of the device.

Assume for example that in a multi-cell converter which operates inaccordance with FIGS. 76A and 76B there are four converter cells, oneconverter cell optimized for “light load conditions” (corresponding tocell 1 _(N1) (2 _(N3)) in FIGS. 76A and 76B), one converter celloptimized for “medium load conditions” (corresponding to cell 1 ₂ (22)in FIGS. 76A and 76B), and one converter cell optimized for “heavy loadconditions” (corresponding to cell 1 ₁ (2 ₁) in FIGS. 76A and 76B). Inthis case, the at least one electronic switch in the light load cell 1_(N1) (2 _(N3)) is designed to have the highest on-resistance, whichwill be referred to as first on-resistance in the following R_(ON1), theat least one electronic switch in the medium load cell 1 ₂ (2 ₂) isdesigned to have a second on-resistance R_(ON2) lower than the firston-resistance R_(ON1), and the at least one electronic switch in theheavy load cell 1 ₁ (2 ₁) is designed to have a third on-resistanceR_(ON3) lower than the second on-resistance R_(ON2). That is,

R _(ON1) >R _(ON2) >R _(ON2).

A ratio R_(ON1):R_(ON2):R_(ON3) is, for example, 1:0.5:0.1. That is, thefirst on-resistance R_(ON1) is two times the second on-resistanceR_(ON2), and 10 times the third on-resistance R_(ON3). Of course, thisis only an example. The ratio between the individual on-resistance mayvary in wide range. Further, not necessarily all the on-resistances ofthe converter cells are different. That is, the at least one electronicswitch in two or more converter cells may be implemented withsubstantially the same on-resistance. However, there are at least twoconverter cells that are designed with different on-resistances. Thatis, the on-resistance of the at least one electronic switch in oneconverter cell is different from the on-resistance of the at least oneelectronic in the other converter cell. “Different” means that theon-resistance in the one converter cell is less than 80% of theon-resistance in the other converter cell.

In case the individual converter cells of the multi-cell converter areimplemented with several electronic switches, such as two switches in aboost topology, four switches in a full-bridge topology, or eightswitches in a DAB topology, there are at least two converter cells inwhich two corresponding electronic switches have differenton-resistances. “Corresponding” means, the electronic switches have thesame position and function in the respective topology. For example, inconverter cells with a boost topology with one half-bridge (as shown inFIG. 12), the high-side switch in one converter cell may have an onresistance which is different from the on-resistance of thecorresponding high-side switch in the other converter cell. If themulti-cell converter is implemented with converter cells that haveseveral electronic switches, there may be other electronic switches thatsubstantially have the same on-resistance in each converter cell.

Another degree of freedom a multi-cell converter topology offers is theratio between the individual DC link voltages V2 ₁-V2 _(N2). In theembodiments explained before, the individual DC link voltages wereassumed to have substantially the same voltage level. However, this isonly an example. According to one embodiment, a multi-cell converterwith an IP topology or IS topology is configured to control the DC linkvoltages at the cell outputs such that they have at least two differentvoltage levels. “Different” means that the voltage level of one group ofDC link voltages is less than 80% of the voltage level of another groupof DC link voltages, wherein each group includes at least one of the DClink voltages explained before. According to another embodiment, amulti-cell converter with an OP or OS topology is configured to controlthe DC link voltages at the cell inputs such that they have at least twodifferent voltage levels. “Different” means that the voltage level ofone group of DC link voltages is less than 80% of the voltage level ofanother group of DC link voltages, wherein each group includes at leastone of the DC link voltages explained before.

As, referring to the explanation above, in a multi-cell converter withan IP or OP topology each converter cell may be configured to controlits associated DC link voltage, different voltage levels of the DC linkvoltages may be obtained by setting the DC link voltage referencesignals in the individual converter cells to different levels. Forexample, in the IP topology shown in FIG. 29, different DC link voltagesV2 ₁-V2 _(N1) may be obtained by setting the DC link voltage referencesignals in the controller 41 of converter cell 1 ₁ and in thecontrollers (not shown) of the other cells to different values. The DClink reference signal of controller 41 is the signal V2 ₁ _(—) _(REF)shown in FIG. 30. This signal and corresponding signals of the othercontrollers may be provided by a central controller (not shown in thedrawings).

Generating DC link voltages with different voltage levels is notrestricted to IP and OP topologies. According to one embodiment, amulti-cell converter with an IS topology is configured to generate DClink voltages V2 ₁-V2 _(N2) with different voltage levels at the celloutputs. One way of operation of such multi-cell converter with an IStopology is explained with reference to FIGS. 79A and 79B below. FIGS.79A and 79B show timing diagrams of one halfwave of a sinusoidal inputvoltage or one full-wave of a rectified sinusoidal input voltage, andtiming diagrams of the total cell input voltage V1 _(TOT). For thepurpose of explanation it is assumed that the multi-cell converterincludes three converter cells that generate different DC link voltagesV2 ₁, V2 ₂, V2 ₃ at their respective cell output. In this embodiment, V2₁>V2 ₂>V2 ₃.

The multi-cell converter may be implemented with a topology as shown inFIG. 12 (when N1=3), the individual converter cells may have one of aboost topology (when the input voltage is a rectified sine voltage) anda full-bridge topology (when the input voltage is a sine voltage). inthe following, the different DC link voltages V2 ₁, V2 ₂, V2 _(N1) willbe referred to as first, second and third DC link voltage, respectively.The converter cells supplying these DC link voltages will be referred toas first, second and third converter cells.

In the embodiments shown in FIGS. 79A and 79B the first, second andthird converter cells are operated in the block mode. That is, based onthe instantaneous level of the voltage V_(IN) only one of the convertercells is operated in a PWM fashion. The other converter cells are eitherin the on-state or the off-state. In the embodiment shown in FIG. 79A,the first converter cells operates in the PWM fashion when the level ofthe input voltage V_(IN) is between 0 and the level of the first DC linkvoltage V2 ₁, the other two converter cells are in the on-state. Whenthe level of the input voltage V_(IN) rises above the level of the firstDC link voltage V2 ₁ the second converter cell starts to operate in thePWM fashion, the first converter cell is in the off-state, and thesecond converter cell is in the on-state. When the level of the inputvoltage V_(IN) rises above a level corresponding to the level of thefirst DC link voltage V2 ₁ plus the level of the second DC link voltageV2 ₂ the third converter cell starts to operate in the PWM fashion, thefirst converter cell is in the off-state, and the second converter cellis in the off-state. The modulation indices of the three converter cellsbased on the voltage level of the input voltage V_(IN) are given inTable 1, below:

TABLE 1 0 < |V_(IN)| ≦ V2₁ V2₁ < |V_(IN)| ≦ V2₁ + V2₂ V2₁ + V2₂ ≦|V_(IN|)| m1 V_(IN)/V2₁ 1 1 m2 0 (V_(IN) − V2₁)/V2₂ 1 m3 0 0 (V_(IN) −V2₁ − V2₂)/V2₃

In Table 1, V_(IN) is the instantaneous level of the input voltage,|V_(IN)| is the absolute value of the instantaneous voltage level of theinput voltage, V2 ₁ is the level of the first DC link voltage, V2 ₂ isthe level of the second DC link voltage, and V2 ₃ is the level of thethird DC link voltage.

After the level of the input voltage V_(IN) has reached the maximumlevel and decreases, first the third converter cell is switched to theon-state in which the cell input power is substantially zero, then thesecond converter cell is switched to the on-state in which the cellinput power is substantially zero, and, finally when the input voltagedecreases to zero, the first converter cell is switched to the on-statein which the cell input power is substantially zero.

The order in which the converter cells begin converting power when thelevel of the input voltage V_(IN) increases is arbitrary. In theembodiment shown in FIG. 77A the first converter cell starts, followedby the second converter cell which is followed by the third convertercell. However, other orders are possible as well. In the embodimentshown in FIG. 79B the third converter cell starts and operates in a PWMfashion until the level of the input voltage V_(IN) reaches the level ofthe third DC link voltage V2 ₃, then the second converter cell operatesin a PWM fashion until the level of the input voltage V_(IN) reaches thelevel of the third DC link voltage V2 ₃ plus the level of the second DClink voltage, and finally the first converter cell operates in the PWMfashion. According to one embodiment, the order in which the convertercells start converting power is different in different halfwaves (orfullwaves). According to one embodiment, the order in which theconverter cells start converting power is dependent on a (desired) powerlevel of the input power P_(IN). In this case, the power level denotesan average power level averaged over one period of the input voltage.If, for example, the average power level is above a predefinedthreshold, the power converter may start in the order shown in FIG. 79Aso that the first converter 1 ₁ has the highest share of the input powerP_(IN). If, for example, the average power level is below the predefinedthreshold, the power converter may start in the order shown in FIG. 79Bso that the third converter 1 ₃ has the highest share of the input powerP_(IN). One embodiment of a main controller 4 configured to operate amulti-cell converter with an IS topology in the way explained withreference to FIGS. 79A and 79B is shown in FIG. 80. This controller isbased on the main controller 4 shown in FIG. 13 and additionallyincludes a block modulation controller 47 which receives the modulationindex m from the modulation index controller 42, and the input voltagesignal V_(IN) _(—) _(M), and which is configured to generate theindividual modulation indices (which are referred to as m₁-m_(N1) inFIG. 80) of the individual converter cells in accordance with Table 1.

In the IS converter explained with reference to FIGS. 79A-81, the timedurations in which the individual converter cells convert power aredifferent. This may result in different cell input powers of theconverter cells. For example, if the peak level of the input voltageV_(IN) is 360V, the first DC link voltage V2 ₁ is 180V, the second DClink voltage V2 ₂ is 120V and the third DC link voltage V2 _(N1) is 60V(so that the total DC link voltage V2 _(TOT) is 360V), and if P_(IN)_(—) _(AVG) is the average input power in one halfwave (or full-wave,respectively) then the average cell input powers P₁ _(—) _(AVG)-P₃ _(—)_(AVG) of the individual converters cells are as follows when theconverter cells are operated as shown in FIG. 79A:

P ₁ _(—) _(AVG)=0.61·P _(IN) _(—) _(AVG)

P ₂ _(—) _(AVG)=0.31·P _(IN) _(—) _(AVG)

P ₃ _(—) _(AVG)=0.08·P _(IN) _(—) _(AVG)

If the converter cells are operated in the order explained withreference to FIG. 79B, the situation is as follows:

P ₁ _(—) _(AVG)=0.39·P _(IN) _(—) _(AVG)

P ₂ _(—) _(AVG)=0.40·P _(IN) _(—) _(AVG)

P ₃ _(—) _(AVG)=0.21·P _(IN) _(—) _(AVG)

In the present embodiment, the average cell input powers aresubstantially balanced, that is the average cell input power of eachcell is substantially ⅓ (0.33) of the average input power P_(IN) _(—)_(AVG) when the converter cells are operated in the order shown in FIG.79B and when the DC link voltages are controlled to have voltage levelsas follows:

V2₁=161V

V2₂=104V

V2₃=95V.

In a multi-cell converter with IS topology which is configured togenerate DC link voltages with different voltage levels, the individualconverter cells 1 ₁-1 _(N1) may be implemented with the same topology.However, the switches in the individual converter cells may be differentin terms of their voltage blocking capability. The “voltage blockingcapability” defines the maximum voltage an electronic switch canwithstand in the off-state (switched off-state) without being damaged.For example, if the electronic switch is implemented as a MOSFET, thevoltage blocking capability is dependent on the specific design of theMOSFET inside a semiconductor chip where active areas of the MOSFET areintegrated. “Different” in this context means that the electronicswitches were intentionally designed to have different voltage blockingcapabilities.

Referring to the above, in an IS converter the voltage blockingcapability of the individual switches implemented therein is higher thanthe level of the corresponding DC link voltage. For example, in theconverter cell 1 ₁ shown in FIG. 12, the high-side and the low-sideswitch 12 _(H), 12 _(L) are each designed to have a voltage blockingcapability that is higher than the associated DC link voltage V2 ₁.Equivalently, in the converter cell 1 _(i) shown in FIG. 24, theindividual switches 17 _(H)-18 _(L) are each designed to have a voltageblocking capability that is lower than the associated DC link voltage V2₁. As the on-resistance of one electronic switches increasesexponentially as the voltage blocking capability increases, it isdesirable to design the individual electronic switches with a voltageblocking capability that is as low as required. Thus, in the embodimentexplained before, the first converter cell is implemented withelectronic switches that have a higher voltage blocking capability thanelectronic switches in the second converter cell, and the secondconverter cell is implemented with electronic switches that have ahigher voltage blocking capability than electronic switches in the thirdconverter cell.

In the example explained above where the DC link voltages V2 ₁-V2 ₃ are180V, 120V and 60V, the first converter cell may be implemented withelectronic switches having a voltage blocking capability of 250V, thesecond converter cell may be implemented with electronic switches havinga voltage blocking capability of 150V, and the third converter cell maybe implemented with electronic switches having a voltage blockingcapability of 80V.

Operating individual converter cells with different DC link voltages isnot restricted to a multi-cell converter with an IS topology. Instead,this kind of operation may be used in a multi-cell converter with an OStopology, such as the multi-cell converter shown in FIG. 34, as well.That is, a multi-cell converter with an OS topology can be designed tocontrol the DC link voltages V2 ₁-V2 ₂ such that these DC link voltagesV2 ₁-V2 ₂ have different voltage levels. Like in the IS converterexplained before, the individual converter cells may be operated in theblock mode. That is, based on the instantaneous voltage level of theoutput voltage only one of the converter cells is operated in a PWMfashion, while the other converter cells are in the on-state or theoff-state.

In FIGS. 79A and 79B the voltages occurring in an OS converter that isconfigured to control DC link voltages V2 ₁-V2 ₂ with different voltagelevels and in which the converter cells are operated in a block mode areindicated in brackets. Besides the waveform of the output voltageV_(OUT), the waveform of the total cell output voltage V3 _(TOT) isshown. It is assumed that the output voltage has the same amplitude asthe input voltage V_(IN) in the embodiment disclosed above and that theDC link voltages V2 ₁-V2 _(N1) have the same voltage levels as in theembodiment explained above.

Like in the IS converter, the converter cells in the OS converter areoperated in a predefined order within one halfwave (or fullwave). In theembodiment shown in FIG. 77A, the converter cell with the highest DClink voltage starts when the level of the output voltage V_(OUT)increases, and in the embodiment shown in FIG. 77B, the converter cellwith the lowest DC link voltage starts when the level of the outputvoltage V_(OUT) increases.

In the embodiment explained with reference to FIGS. 79A and 79, thevoltage levels of the individual DC link voltages V2 ₁-V2 _(N2) may becontrolled by the IS converter and the OS converter, respectively. Thatis, the power converter not only controls the total DC link voltage V2_(TOT) but also the different levels of the individual DC link voltagesV2 ₁-V2 _(N2). According to another embodiment, a further power controlsthe levels of the individual DC link voltages. For example, in case ofthe IS converter explained above, a further power converter may beconnected to the DC link capacitors and receive power from the ISconverter. According to one embodiment, the further power converter hasan OP topology with a plurality of converter cells, wherein eachconverter cell of the further power converter controls the DC linkvoltage across one respective DC link capacitor. For example, in case ofthe OS converter explained above, a further power converter may beconnected to the DC link capacitors and provide power to the OSconverter. According to one embodiment, the further power converter hasan IP topology with a plurality of converter cells, wherein eachconverter cell of the further power converter controls the DC linkvoltage across one respective DC link capacitor.

In the explanation above, it has been assumed that V_(IN)=m·V2 _(TOT) inan IS converter, and V_(OUT)=m·V2 _(TOT) in an OS converter,respectively. However, there may be cases where m·V2 _(TOT), which willbe referred to as V_(REF) in the following and which, more generally,can be expressed as

V _(REF) =m·V2_(TOT)=Σ_(i) ^(N2) m _(i) ·V2_(i)  (29),

is not exactly the input voltage V_(IN) and the output voltage V_(OUT),respectively. In general, there is a phase shift between theinput/output voltage V_(IN)/V_(OUT) and m·V2 _(TOT), which may beseveral degrees and which is dependent on an inductance of the inductor15 explained above. In those cases explained above, where the operationof the power converter has been explained to dependent on one of theinput voltage and the output voltage, respectively, such as in theembodiments shown in FIGS. 71, 74, and 79A-79B, operation of the powerconverter can be dependent on V_(REF) instead of V_(IN) and V_(OUT) aswell, in particular in those cases where the inductor 15 has arelatively high inductance.

For example, in the embodiments shown in FIGS. 71 and 74 the twoconverter cells may be connected in parallel or in series dependent onV_(REF) instead of dependent on one of V_(IN) and V_(OUT), respectively.In the embodiment shown in FIGS. 79A and 79B the voltage thresholdswhere the converter cells change their mode of operation may be comparedwith V_(REF) instead of V_(IN) and V_(OUT), respectively. In this case,the block modulation controller 47 shown in FIG. 80 receives the DC linkvoltage signals (shown in dashed lines) in order to calculate V_(REF).

However, using V_(REF) instead of V_(IN) and V_(OUT) to take thedecision if the operation of the power converter is to be changed doesnot change the general behavior so that in the description above V_(IN)and V_(OUT) were used instead of V_(REF) to describe the operation ofthe power converter. However, the operation based on VIN and VOUT,respectively is to be understood to include an operation based on VREFas well. That is, for example, in FIGS. 71, 74, 79A-79B V_(IN) andV_(OUT) may be replaced by V_(REF).

Another degree of freedom in a multi-cell converter is the specificdesign of the half-bridges in those types of converter cells whichinclude a half-bridge. Those types of converter cells are, for example,the converter cells with a boost topology as shown in FIG. 12, with afull bridge topology as shown in FIG. 24, and with a buck topology asshown in FIG. 32B. FIG. 81 shows a half-bridge with a high-side switchHS and a low-side switch LS. This half-bridge represents any half-bridgein those converter cells with a boost topology or a totem pole topologyexplained before In the multi-cell converters with converter cells ofthis type, there are operation scenarios in which the half-bridge isoperated in a PWM mode. This is explained in FIG. 82 which shows timingdiagrams of a drive signal SLS of the low-side switch LS, and of a drivesignal SHS of the high-side switch HS in one drive cycle having aduration Tp. Referring to FIG. 82, the low-side switch LS is switched onfor an on-period Ton, while the high-side switch is off. After thelow-side switch LS has been switched off, the high-side switch HS isswitched on. An on-state of the low-side switch is represented by anon-level of the corresponding drive signal SLS, and an on-level of thehigh-side switch HS is represented by an on-level of the correspondingdrive signal SHS in FIG. 82 (in which, just for the purpose ofillustration) on-levels are represented by a high-level and off-levelsare represented by a low-level. There may be a delay time (dead time)between the time when the low-side switch LS switches off and thehigh-side switch HS switches on. However, this delay time is not shownin FIG. 82.

In the embodiment shown in FIG. 81, the two switches HS, LS are drawn asMOSFETs, in particular as n-type MOSFETs. However, other type oftransistors such as, for example, IGBTs, BJTs, JFETs, or the like, maybe used as well. Independent of the specific type of electronic switchused to implement the high-side and the low-side switch, losses(conduction losses) occur when the respective switch HS, LS is in theon-state. The conduction losses of one switch are dependent on theelectrical resistance of the switch in the on-state. This electricalresistance will be referred to as on-resistance R_(ON) in the following.In the multi-cell converters explained hereinbefore, the individualconverter cells may be operated in a continuous current mode (CCM). Inthis operation mode, a current through the converter cell does notdecrease to zero in one drive cycle (except for the time in which therespective input voltage V_(IN) or output voltage V_(OUT) of themulti-cell converter is zero). For the purpose of explanation it mayfurther be assumed that the current through the low-side switch LSduring the on-time Ton is substantially equal the current through thehigh-side switch HS during the off-time Toff. The off-time Toff is thetime between switching off the low-side switch LS and the end of thedrive cycle. Losses that occur in one of the high-side switch HS and thelow-side switch LS increases as the duration of the on-time of therespective switch increases. If the high-side switch HS and the low-sideswitch LS have substantially the same on-resistance R_(ON), and when theduty cycle d=0.5 substantially the same losses occur in the high-sideswitch HS and the low-side switch LS because each of the switches is inthe on-state for a duration that substantially equals 0.5·Tp at d=0.5

The on-resistance R_(ON) of one switch is substantially reciprocalproportional to a chip area on a semiconductor chip in which therespective switch is implemented. If, for example, there is an overallchip area A available to implement the first switch HS and the secondswitch LS, and if each of the two switches HS, LS are implemented withsubstantially the same chip area, namely 0.5·A, then the two switchesHS, LS have substantially the same on-resistance R_(ON). If the twoswitches are designed to have substantially the same on-resistancesR_(ON), the overall conduction losses, which are the losses occurring inthe high-side switch HS and the low-side switch LS are independent ofthe duty cycle d. If the duty cycle d is different from 0.5, the overallconduction losses can be reduced by designing the two switches to havedifferent on-resistances. This is explained with reference to FIG. 79.In this context, “different” means that the electronic switches wereintentionally designed to have different on-resistances. How theon-resistance of an electronic switch can be adjusted is explainedherein before.

FIG. 83 shows the overall conduction losses P_(LOSS)(a,d) dependent onthe duty cycle d relative to the overall conduction lossesP_(LOSS)(0.5,d) at equal chip areas of the HS and LS switch fordifferent designs of the high-side switch HS and the low-side switch LS.The overall conduction losses are the losses occurring in the high-sideand the low-side switch HS, LS in one drive cycle. In FIG. 83, “a”denotes the chip area of the low-side switch LS relative to the overallchip area used for implementing the high-side switch HS and the low-sideswitch LS. If, for example, a=0.1 then the chip area of the low-sideswitch LS is only 0.1 times the overall chip area, while the chip areaof the high-side switch is 0.9 times the overall chip area.Consequently, the on-resistance of the low-side switch is 9 times theon-resistance of the high-side switch. The dashed line in FIG. 83represents the case in which the high-side switch and the low-sideswitch are designed with the same chip area, which is 0.5 times theoverall chip area. A half-bridge in which the high-side switch HS andthe low-side switch LS are designed with the same chip area will bereferred to as a half-bridge with a symmetric design (symmetrichalf-bridge). Equivalently, a half-bridge designed with electronicswitches HS, LS having different chip areas will be referred to ashalf-bridge with an asymmetric design (asymmetric half-bridge).

As can be seen from FIG. 83, a half-bridge with an asymmetric design canbe superior to a half-bridge with a symmetric design (which isrepresented by the dashed line labeled with 0.5 in FIG. 83) if the dutycycle is in a certain range. For example, a half-bridge with anasymmetric design in which a=0.2 has lower losses than a half-bridgewith a symmetric design if the duty cycle is below d=0.2. In general,for a<0.5, the asymmetric design offers lower losses if d<a. If a>0.5,the asymmetric design offers lower losses if d>a.

According to one embodiment, a multi-cell converter with an IS topologyor an OS topology and including converter cells with at least onehalf-bridge, such as boost converter cells or totem pole convertercells, includes at least one converter cell with an asymmetrichalf-bridge. In those multi-cell converters, the modulation index and,therefore, the duty cycle of the individual converter cells may varyover a relatively large range during one halfwave of a sinusoidal inputvoltage (output voltage). The asymmetric design of at least onehalf-bridge in at least one converter cell and the varying duty cycleoffers the possibility to operate the converter cell with the asymmetrichalf-bridge at a duty cycle in which the asymmetric design is superiorto the symmetric design. This is explained with reference to FIG. 84herein below.

FIG. 84 illustrates one method of operating a multi-cell converter withan IS topology or with an OS topology. In particular, FIG. 84illustrates a method for calculating modulation indices m₁-m_(N1) of theindividual converter cells which may have a boost topology or a totempole topology. The method shown in FIG. 80 applies to a first powerconverter 10 with N1 converter cells. However, this method appliesequivalently to a second power converter with N3 converter cells.Referring to the explanation above, a multi-cell converter with IStopology may be operated such that the instantaneous level of the inputvoltage V_(IN) substantially corresponds to the product of themodulation index m and the total DC link voltage V2 _(TOT) (a multi-cellconverter with OS topology may be operated such that the instantaneouslevel of the output voltage V_(OUT) substantially corresponds to theproduct of the modulation index m and the total DC link voltage V2_(TOT)).

Referring to the above, the instantaneous voltage level of the inputvoltage V_(IN) of an IS converter may be tracked by the total cell inputvoltage V1 _(TOT) by operating the individual converter cells with thesame modulation index m at one time. However, it is also possible tooperate the individual converter cells with different modulationindices. In this case, the individual converter cells are to be operatedsuch that V_(IN)=m₁·V2 ₁+m₂·V2 ₂+ . . . +m_(N1)·V2 _(N2). The pluralityof modulation indices m₁-m_(N1) in this equation can be regarded as amodulation index vector. It can be shown that the equation above is metby a plurality of different modulation index vectors. If, for example,the converter cell 1 ₁ receiving modulation index m₁ has a highefficiency at a high modulation index (which corresponds to a low dutycycle), then the modulation index vector may be calculated such that m₁is high, where other modulation indices may be lower. The individualmodulation indices m₁-m_(N1) obtained through this method may be appliedto the individual converter cells (1072).

FIG. 85 shows one embodiment of a main controller 4 which is configuredto control a first power converter 10 with an IS topology and with atleast one converter cell having an asymmetric half-bridge. This mainconverter 4 shown in FIG. 81 is based on the main converter 4 shown inFIG. 13 and is different from this main converter 4 in that itadditionally includes a converter cell controller 46 which is configuredto generate the modulation indices m₁-m_(N1) in accordance with themethod explained with reference to FIG. 80.

FIG. 86 shows a corresponding main controller 5 of a second powerconverter 20 with an OS topology. This main controller 5 is based on themain controller 5 shown in FIG. 35 and is different from this maincontroller shown in FIG. 35 in that it additionally includes a convertercell controller 56 a converter cell controller 56 which calculates themodulation indices m₁-m_(N3) received by the individual converter cells2 ₁-2 _(N3) in accordance with the method explained with reference toFIG. 80 above.

Alternatively or additionally to operating individual converter cells ofa multi-cell converter, in particular a multi-cell converter with an ISor OS topology, at different modulation indices in order to operateindividual converter cells close to their optimum operation point, theswitching frequency (which has been referred to as fp above) may bevaried. That is, at least two converter cells of a multi-cell converterwith an IS or OS topology may be operated with different switchingfrequencies in the PWM mode. The modulation index may be the same forthe two converter cells, or may be different. The two converter cellsmay be operated in the PWM mode at the same time or at different times.Nevertheless, by operating the at least two converter cells in the PWMmode with different switching frequencies, the efficiency curves of thetwo converter cells are different, so that for, example, the convertercell with the higher switching frequency may have the maximum efficiencyat a lower power level than the converter cells with the lower switchingfrequency. According to one embodiment, the switching frequency of theconverter cell with the higher switching frequency is at least two timesthe switching frequency of the converter cell with the lower switchingfrequency.

FIG. 87 shows one embodiment of a full-bridge which includes twohalf-bridges HB1, HB2 wherein each half-bridge HB1, HB2 includes ahigh-side switch HS1. HS2 and a low-side switch LS1, LS2. Each of thesehigh-side and low-side switches HS1, LS2 includes at least one siliconMOSFET. In the embodiment shown in FIG. 83, these MOSFETs are n-typeMOSFETs, however, p-type MOSFETs may be used as well. Instead of onlyone MOSFET, each of these switches may include two or more MOSFETs whichhave their load paths connected in parallel and which are switched onand off simultaneously.

The full-bridge shown in FIG. 87 represents the full-bridge in anyconverter cell with a full-bridge (totem pole) topology in any of the ISor OS multi-cell converters explained hereinbefore. Referring to FIG. 25and the corresponding description, one of these half-bridges is operatedin a PWM mode, such as a PWM mode explained with reference to FIGS. 81and 81 hereinbefore. Referring to FIG. 87, silicon MOSFETs include aninternal diode which is explicitly drawn in FIG. 88. This diode is oftenreferred to as body diode. If one of these half-bridges is operated in aPWM mode such that there is a delay time between switching off one ofthe two switches and switching on the other one of the two switches, thebody diode of the other switch becomes conducting. This is explainedwith reference to the half-bridge 17 shown in FIG. 24.

If the low-side switch 17 _(L) is conducting, the input current I0 _(i)flows through the low-side switch 17 _(L). If the low-side switch 17_(L) switches off the input current I0 _(i) (driven by the at least oneinductor of the multi-cell power converter circuit) flows through thediode connected in parallel with high-side switch 17 _(H). This diodeshown in FIG. 24 can be formed by the body diode of a MOSFET when thehigh-side switch 17 _(H) is implemented as a MOSFET. A current flowsthrough the diode until the high-side switch 17 _(H) switches on. At theend of one drive cycle 17 _(H) switches on and the low-side switch 17_(L) again switches on. There may be a delay time between switching offthe high-side switch 17 _(H) and switching on the low-side switch 17_(L), so that the input current I0 _(i) continuous to flow through thediode of the high-side switch 17 _(H) until the low-side switch 17 _(L)switches on.

When the diode of the high-side switch 17 _(H) conducts the inputcurrent I0 _(i) electrical charge is stored in the diode. Thiselectrical charge has to be removed from the diode before the diodeblocks. This effect of removing electrical charge from a bipolar diodeis commonly known as reverse recovery.

The electrical charge stored in the body diode of a MOSFET, when thebody diode is conducting, is, inter alia, dependent on a so-calledoutput capacitance of the MOSFET. This output capacitance and, thecharge stored in the body diode, increases as the voltage blockingcapability of the MOSFET increases, wherein the output capacitanceincreases exponentially. That is the output capacitance is a function ofV_(B) _(c) , with b>1, where V_(B) denotes the voltage blockingcapability. By virtue of this relatively high output capacitanceconverters silicon MOSFETs were not considered suitable to implement theswitches in a power converter with a totem pole topology. In thiscontext reference is made Zhou et al.: “99% Efficiency True-BridgelessTotem-Pole PFC Based on GaN HEMTs”.

In a multi-cell converter with an IS or OS topology, however, theindividual switches can be designed with a voltage blocking capabilitywhich is lower than the DC link voltage. If, for example, the total DClink voltage is 600V and a conventional power converter (with a PFCfunctionality) is employed, the converter is to be implemented withswitches having a voltage blocking capability of 600V. In the IS or OSconverters explained above, the switches of one converter cell may beimplemented with a voltage blocking capability which only corresponds tothe voltage level of the respective DC link voltage. For example, ifthere are N1=4 or N3=4 converter cells 1 ₁-1 _(N1) and 2 ₁-2 _(N3),respectively, it may be sufficient to design the individual switcheswith a voltage blocking capability of 150V (=600V/4). In case there areN1=10 or N3=10 only a voltage blocking capability of only 60V (=600V/10)may be sufficient.

In an IS or OS converter, the overall on-resistance is N1 (or N3) timesthe on-resistance of one switch, so that the on-resistance increaselinearly as the number of converter cells increases. However, theoverall reverse recovery charge stored in the switches of the individualconverter cells decrease exponentially. This is shown by way of anexample below. In a silicon MOSFET there is a figure of merit (FOM)which describes the relationship between the on resistance and thecharge which has to be removed from the MOSFET when the body diodeswitches from the forward biased to the reverse biased state, namelyR_(ON)·Q_(REV) _(—) _(REC) (Q_(REV) _(—) _(REC) is often referred to asQrr+Qoss, where Qoss is the charge stored in an output capacitance, andQrr is the charge stored in the diode when switched from a forwardcurrent to a reverse current The on-resistance can be decreased bydesigning the MOSFET with a larger chip area, wherein the on-resistanceR_(ON) is substantially inversely proportional to the chip area.However, as Q_(REV-REC) is substantially proportional to the chip area,the FOM defined above is substantially independent of the chip area andmainly dependent on the voltage blocking capability and the specificdesign.

A MOSFET with a voltage blocking capability of 600V of the CoolMOS™ CFD2series of Infineon Technologies AG, Munich has an FOM of about 78000(7.8E4). A MOSFET with a voltage blocking capability of 60V of theOptiMOS series of the same supplier has an FOM of only 346. The overallFOM of 10 series connected converter cells is 3460, which is 22 timesbetter than the FOM of only one MOSFET with a voltage blockingcapability of 600V.

Thus, a multi-cell converter with a series connection of several, suchas 4, 6, 10 or more, converter cells has a reverse recovery behaviorwhich is competitive.

Referring to the disclosure in context with FIG. 1, the power convertercircuit includes at least one multi-cell converter. That is each type offirst power converter 10 with a multi-cell topology explained before maybe coupled to a second power converter which does not have a multi-celltopology, or may be used alone without a second power converter.Equivalently, each type of second power converter 20 with a multi-celltopology explained before may be coupled to a first power converterwhich does not have a multi-cell topology, or may be used alone withouta first power converter. This is explained by way of two examples withreference to FIGS. 88 and 89 below.

FIG. 88 shows one embodiment of a power converter circuit in which thesecond power converter 20 is a multi-cell converter of any of the typesexplained before. The first power converter is a single cell converter.That is, the first power converter includes only one converter cell 1 ₁which is configured to receive power from the input IN1, IN2 and toprovide power to the plurality of DC link capacitors 11 ₁-11 _(N1) whichare connected in series at the cell output of the converter cell 1 ₁.The converter cell 1 ₁ may have one of a step-up and a step-downcharacteristic. That is the total DC link voltage may be higher or lowerthan a (peak) level of the input voltage.

FIG. 89 shows one embodiment of a power converter circuit which includesa second converter 20 of any of the types explained before. In thisembodiment, there is no further power converter (no first converter).The individual DC link capacitors 2 ₁-2 _(N2) couple the second powerconverter 20 to a DC power source 9 with a plurality of power sourcecells 9 ₁-9 _(N2), wherein each power source cell is connected to one DClink capacitor 2 ₁-2 _(N2). Examples of the power source cells include,but are not restricted to, batteries, photovoltaic (PV) panels, fuelcells, or the like. According to one embodiment, the second powerconverter 20 includes an OS topology and a PFC capability and isconfigured to supply power to an AC power grid.

FIG. 90 shows one embodiment of a power converter circuit with a firstpower converter 10 and a second power converter 20. The second powerconverter includes a plurality of converter cells 2 ₁-2 _(N3) eachreceiving power from the first converter 10 and the associated DC linkcapacitor 11 ₁-11 _(N2). The topology of the second converter 20 isdifferent from each of the second converter topologies explained abovein that the cell output of each of the plurality of converter cells 2₁-2 _(N3) is connected to one of a plurality of loads Z₁-Z_(N3) whichare supplied by the second converter 20. Thus, the cell outputs of theconverter cells 2 ₁-2 _(N3) are not connected (neither connected inseries nor in parallel). According to one embodiment, the loadsZ₁-Z_(N3) are Dc loads so that the individual converter cells 2 ₁-2_(N3) are DC/DC converter cells. The first converter may have an IStopology and a PFC capability.

According to one embodiment, the first converter 10 is configured toreceive the input power from a medium voltage grid. Using the powerconverter circuit shown in FIG. 89 DC loads, such as the loads Z₁-Z_(N3)can directly be supplied from a medium voltage grid without having totransform the medium AC voltage to a low AC voltage. Dependent on thespecific type of the medium voltage grid, the peak input voltage can beup to several 10 kV. However, by virtue of the IS topology in the firstconverter 10 semiconductor switches with a voltage blocking capabilityof much less than the peak input voltage can be used in the convertercells of the first converter 10. In this embodiment more than 10 and upto several 10 converter cells may be used in the first converter 10 and,consequently, the second converter 20. The “voltage blockingcapability”defines the maximum voltage an electronic switch canwithstand in the off-state (switched off-state) without being damaged.

The first and second power converters 10, 20 explained before may becombines in various different ways so as to obtain power convertercircuits for many different applications in the field of AC/DC, DC/AC,or DC/DC power conversion. Some of these applications are explainedbelow. In these applications, the specific design of the first powerconverter 10 and the second power converter 10, respectively, can beselected based on different parameters such as, for example, the (peak)level of the input voltage, and the (peak) level of the output voltage.An IS topology may be used when the level of the input voltage isrelatively high, such as higher than 100V, an IP topology may be usedwhen the voltage level is lower. Equivalently, an OS topology may beused when the level of the output voltage is relatively high, such ashigher than 100V, an OP topology may be used when the voltage level islower. In the design of the power converter circuit, the number ofconverter cells in the first power converter circuit 10 and the secondpower converter circuit, respectively can be dependent on the peak inputvoltage and may be higher the higher the peak input voltage is.

An AC/DC power converter circuit may be configured to receive a lowvoltage from a low voltage power grid, or a medium voltage from a mediumvoltage power grid. A low voltage power grid supplies a sine voltagewith 110V_(RMS) or 220V_(RMS) (so that a peak voltage is about 155V or310V, respectively). A medium voltage grid supplies a sine voltage witha peak voltage of several kilovolts (kV), up to 10 kV. The AC/DC powerconverter circuit may include a first power converter 10 with whichcontrols the DC link voltages V2 ₁-V2 _(N2), and a second powerconverter 20 which controls the output voltage V_(OUT).

A DC/AC power converter circuit may be configured to receive a DC powerfrom a DC voltage source and to supply an AC power voltage to an ACpower grid. According to one embodiment, the DC power source includes asolar panel. According to one embodiment, the DC power source includes ahigh-voltage, direct current (HVDC) power transmission grid. The powergrid supplied by the DC/AC power converter circuit may be a low voltageor a medium voltage power grid. The DC/AC power converter circuit mayinclude a first power converter which controls one of the input currentI_(IN) and the input voltage V_(IN), and a second power converter 20which controls the DC link voltages V2 ₁-V2 _(N2) output voltageV_(OUT).

Basically, each of the multi-cell converters explained herein before maybe implemented in a power converter circuit with together with anothermulti-cell converter, with a single cell converter, or alone, that ist,without another power converter. In case there is another (multi-cell orsingle cell) converter, the other converter, dependent on the specifictopology, either supplies power to the multi-cell converter and the DClink capacitors, respectively, or receives power from the multi-cellconverter and the DC link capacitors, respectively.

Above, several types of multi-cell converters, power converter circuitswith at least one multi-cell converters, and different ways of operationof such multi-cell converters and power converter circuits aredisclosed. Of course, the aspects explained above may be combined witheach other. Some of these aspects are summarized below.

Some of the aspects explained above relate to:

A1. A method including: converting power by a power converter includinga plurality of converter cells; and selectively operating at least oneconverter cell of the plurality of converter cells in one of an activeand an inactive mode based on a level of a power reference signal.

A2. The method of item A1, further including: operating at least oneother converter cell of the plurality of converter cells in an activemode when the at least one converter cell is operated in the inactivemode.

A3. The method of item A2, wherein the at least one other converter cellof the plurality of converter cells includes the remainder of theplurality of converter cells.

A4. The method of one of items A1-A3, wherein operating the at least oneconverter cell in the inactive mode includes converting zero power bythe at least one converter cell.

A5. The method of one of items A1-A4, wherein operating the at least oneconverter cell in one of the active mode and the inactive mode based onthe level of the power reference signal includes: operating the at leastone converter cell in the inactive mode when the level of the powerreference signal is below a predefined threshold.

A6. The method of one of items A1-A5, wherein operating the at least oneconverter cell in the active mode includes operating at least one switchin the at least one converter cell in a PWM (Pulse-Width Modulated)fashion at a switching frequency, and wherein operating the at least oneconverter cell in the inactive mode includes operating the at least oneconverter cell in the inactive mode for a time duration which is atleast ten times a reciprocal of the switching frequency.

A7. The method of one of items A1-A6, wherein operating the at least oneconverter cell in the inactive mode includes setting a number ofconverter cells which are operated in the inactive mode based on thelevel of the power reference signal, wherein the number increases as thelevel decreases.

A8. The method of one of items A1-A7, wherein each of the plurality ofconverter cells includes a cell input configured to receive a cellvoltage and a cell output, and wherein the cell outputs of the pluralityof converter cells are connected in parallel, wherein a parallel circuitwhich includes the cell outputs of the plurality of converter cells iscoupled to an output of the power converter.

A9. The method of item A8, wherein the plurality of converter cells isconfigured to control one of an output voltage, and an output current atthe output of the power converter based on the power reference signal,wherein the power reference signal includes an output current referencesignal.

A10. The method of item A9, wherein operating the at least one of theplurality of converter cells in one of the active mode and the inactivemode includes operating the at least one of the plurality of convertercells in the inactive mode when a level of the output current referencesignal is below a predefined current threshold.

A11. The method of item A8, wherein operating the at least convertercell in one of the active mode and the inactive mode includes: selectinga number K of converter cells to be operated in the inactive mode basedon the level of the output current reference signal; identifying those Kconverter cells that receive the lowest cell input voltages; andoperating the identified converter cells in the inactive mode.

A12. The method of item A10, further including: repeating the selecting,identifying, and operating.

A13. The method of item A12, wherein the repeating includes repeatingregularly.

A14. The method of item A12, wherein the repeating includes repeatingwhen a voltage level of the input voltage of one of the plurality ofsecond converter cells falls below a predefined voltage threshold.

A15. The method of one of items A1-A14, wherein the plurality ofconverter cells is configured to control the output voltage to besubstantially constant.

A16. The method of one of items A1-A14, further including: providing acell input voltage at the cell input of each of the plurality ofconverter cells by another power converter.

A17. The method of item A16, wherein the other power converter includesat least one converter cell.

A18. The method of item A17, wherein the power converter and the otherpower converter are linked by a plurality of capacitors.

A19. The method of item A18, wherein each of the cell input voltages isa voltage across a respective one of the plurality of capacitors.

A20. The method of one of items A16-A19, wherein the other powerconverter is configured to control a sum of the cell input voltages.

A21. The method of item A1, wherein each of the plurality of convertercells includes a cell input and a cell output configured to provide acell output voltage, and wherein the cell inputs of the plurality ofconverter cells are connected in parallel, wherein a parallel circuitwhich includes the cell inputs of the plurality of converter cells iscoupled to an input of the power converter.

A22. The method of item A21, wherein the plurality of converter cells isconfigured to control one of an input voltage, and an input current atthe input of the power converter based on the power reference signal,wherein the power reference signal includes an input current referencesignal.

A23. The method of item A22, wherein operating the at least one of theplurality of converter cells in one of the active mode and the inactivemode includes operating the at least one of the plurality of convertercells in the inactive mode when a current level of the input currentreference signal is below a predefined current threshold.

A24. The method of item A21, wherein operating the at least convertercell in one of the active mode and the inactive mode includes: selectinga number K of converter cells to be operated in the inactive mode basedon the level of the input current reference signal; identifying those Kconverter cells that have the highest cell output voltages; andoperating the identified converter cells in the inactive mode.

A25. The method of item A24, further including: repeating the selecting,identifying, and operating.

A26. The method of item A25, wherein the repeating includes repeatingregularly.

A27. The method of item A25, wherein the repeating includes repeatingwhen a voltage level of the input voltage of one of the plurality ofsecond converter cells rises above a predefined voltage threshold.

A28. The method of one of items A22-A27 wherein the plurality ofconverter cells is configured to control the input voltage to besubstantially constant.

A29. The method of one of items A21-A28, further including: receiving acell output voltage at the cell output of each of the plurality ofconverter cells by another power converter.

A30. The method of item A29, wherein the other power converter includesonly one converter cell.

A31. The method of one of items A29-A30, wherein the power converter andthe further power converter are linked by a plurality of capacitors.

A32. The method of item A31, wherein each of the cell output voltages isa voltage across a respective one of the plurality of capacitors.

A33. A method including: receiving a periodic voltage by a powerconverter which includes a plurality of converter cells; and selectivelyoperating at least one converter cell of the plurality of convertercells in one of an active and an inactive mode based on a change of avoltage level of the periodic voltage such that within one period of theperiodic voltage, the number of active converter cells changes as thevoltage level of the periodic voltage changes.

A34. The method of item A33, wherein selectively operating the at leastone converter cell in one of the active and the inactive mode includesoperating the at least one converter cell such that the number of activeconverter cells increases as the voltage level of the periodic voltageincreases.

A35. The method of one of items A33-A34, wherein selectively operatingthe at least one converter cell in one of the active and the inactivemode includes operating the at least one converter cell such that thenumber of active converter cells decreases as the voltage level of theperiodic voltage decreases.

A36. The method of one of items A33-A35, wherein each of the pluralityof converter cells includes a cell input and a cell output, wherein thecell inputs of the plurality of converter cells are connected inparallel, wherein a parallel circuit which includes the cell inputs iscoupled to an input of the power converter, and wherein the cell outputof each converter cell is coupled to a respective capacitor.

A37. The method of one of items A33-A36, wherein each of the pluralityof converter cells includes a cell input and a cell output, wherein thecell outputs of the plurality of converter cells are connected inparallel, wherein a parallel circuit which includes the cell inputs iscoupled to an input of the power converter, and wherein the cell inputof each converter cell is coupled to a respective capacitor.

A38. The method of one of items A34-A37, wherein in each of a pluralityof successive periods or half-periods two or more converter cells areactivated as the voltage level of the input voltage increases, andwherein an order in which the two or more converter cells are activatedis different in at least two different periods or half-periods.

A39. The method of one of items A35-A38, wherein in each of a pluralityof successive periods or half-periods two or more converter cells aredeactivated as the voltage level of the input voltage decreases, andwherein an order in which the two or more converter cells aredeactivated is different in at least two different periods orhalf-periods.

A40. The method of one of items A33-A39, wherein operating the at leastone converter cell in the inactive mode includes converting zero powerby the at least one converter cell.

A41. A power converter circuit including a power converter, wherein thepower converter includes: a plurality of converter cells; and acontroller configured to operate at least one converter cell of theplurality of converter cells in one of an active and an inactive modebased on a level of a power reference signal.

A42. The power converter circuit of item A41, wherein each of theplurality of converter cells includes a cell input configured to receivea cell input voltage and a cell output, and wherein the cell outputs ofthe plurality of converter cells are connected in parallel, wherein aparallel circuit which includes the cell outputs of the plurality ofconverter cells is coupled to an output of the power converter.

A43. The power converter circuit of item A42, wherein the controller isconfigured to control one of an output voltage, and an output current atthe output of the power converter based on the level of the powerreference signal, wherein the power reference signal includes an outputcurrent reference signal

A44. The power converter circuit of item A43, wherein the controller isconfigured to operate the at least one of the plurality of convertercells in the inactive mode when a level of the output current referencesignal is below a predefined current threshold.

A45. The method of one of items A42-A44, wherein the controller isconfigured: to select a number K of converter cells to be operated inthe inactive mode based on the level of the output current referencesignal; to identify those K converter cells that receive the lowest cellinput voltages; and to operate the identified converter cells in theinactive mode.

A46. The power converter circuit of item A38, wherein each of theplurality of converter cells includes a cell input and a cell outputconfigured to provide a cell output voltage, and wherein the cell inputsof the plurality of converter cells are connected in parallel, wherein aparallel circuit which includes the cell inputs of the plurality ofconverter cells is coupled to an input of the power converter.

A47. The power converter circuit of item A46, wherein the controller isconfigured to control one of an input voltage and an input current atthe input of the power converter based on the power reference signal,wherein the power reference signal includes an input current referencesignal.

A48. The power converter circuit of item A47, wherein the controller isconfigured to operate the at least one of the plurality of convertercells in the inactive mode when a level of the input current referencesignal is below a predefined current threshold.

A49. The power converter circuit of one of items 46-A48, wherein thecontroller is configured: to select a number K of converter cells to beoperated in the inactive mode based on the level of the input currentreference signal; to identify those K converter cells that have thelowest cell output voltages; and to operate the identified convertercells in the inactive mode.

A50. A power converter circuit, including: a power converter whichincludes a plurality of converter cells and is configured to receive aperiodic voltage; and a controller, wherein the controller is configuredto selectively operate at least one converter cell of the plurality ofconverter cells in one of an active and an inactive mode based on achange of the periodic voltage such that within one period of theperiodic voltage, the number of active converter cells changes as avoltage level of the periodic voltage changes.

A51. The power converter circuit of item A50, wherein the controller isconfigured to selectively operate the at least one converter cell in oneof the active and inactive mode such that within one period of theperiodic voltage, the number of active converter cells increases as thevoltage level of the periodic voltage increases.

A52. The power converter circuit of one of items A50-A51, wherein thecontroller is configured to selectively operate the at least oneconverter cell in one of the active and inactive mode such that withinone period of the periodic voltage, the number of active converter cellsdecreases as the voltage level of the periodic voltage decreases.

B1. A power converter circuit including: a plurality of converter cells,wherein at least a first converter cell of the plurality of convertercells has a first operational characteristic, and wherein at least asecond converter cell of the plurality of converter cells has a secondoperational characteristic different than the first operationalcharacteristic.

B2. The power converter circuit of item B1, further including: aplurality of capacitors, each associated with one of the plurality ofconverter cells, wherein the power converter is configured to control avoltage across each of the plurality of capacitors, and wherein thefirst operational parameter includes a first voltage level of a voltageacross a first capacitor associated with the first converter cell, andthe second operational parameter includes a second voltage level of avoltage across a second capacitor associated with the second convertercell.

B3. The power converter circuit of item B2, wherein the first voltagelevel is less than 80% of the second voltage level.

B4. The power converter circuit of one of items B1-B3, wherein the powerconverter is configured to control a voltage across each of theplurality of capacitors such that the voltages across the plurality ofcapacitors are mutually different.

B5. The power converter circuit of one of items B1-B4, wherein each ofthe plurality of converter cells includes a cell input and a celloutput, wherein the cell inputs of the plurality of converter cells areconnected in series, wherein a series circuit which includes the cellinputs of the plurality of converter cells is coupled to an input of thepower converter, and wherein the cell output of each of the plurality ofconverter cells is connected to a respective one of the plurality ofcapacitors.

B6. The power converter circuit of one of items B1-B5, wherein the powerconverter is configured to receive a periodic voltage at the input ofthe power converter, and to operate each of the converter cells in oneof three different operation modes based on a voltage level of the inputvoltage.

B7. The power converter of item B6, wherein the three differentoperation modes include: an on-mode; an off-mode; and a PWM (Pulse-WidthModulation) mode.

B8. The power converter of one of items B6-B7, wherein each of theconverter cells includes at least one electronic switch, whereinoperating each of the converter cell in one of the three differentoperation modes includes operating each of the converter cell insuccessive drive cycles, wherein in the on-mode the at least oneelectronic switch is in an on-state throughout each drive cycle, whereinin the off-mode the at least one electronic switch is in an off-statethroughout each drive cycle, and wherein in the PWM mode, the at leastone electronic switch is in the on-state for an on-period and in theoff-state for an off-period in each drive cycle.

B9. The power converter circuit of one of items B1-B8, wherein each ofthe plurality of converter cells includes a cell input and a celloutput, wherein the cell outputs of the plurality of converter cells areconnected in series, wherein a series circuit which includes the celloutputs of the plurality of converter cells is coupled to an output ofthe power converter, and wherein the cell input of each converter cellis connected to a respective one of the plurality of capacitors.

B10. The power converter circuit of item B9, wherein the power converteris configured to receive a periodic voltage at the output of the powerconverter, and to operate each of the converter cells in one of threedifferent operation modes based on a voltage level of the input voltage.

B11. The power converter of item B10, wherein the three differentoperation modes include: an on-mode; an off-mode; and a PWM mode.

B12. The power converter of item B11, wherein each of the convertercells includes at least one electronic switch, wherein operating each ofthe converter cell in one of the three different operation modesincludes operating each of the converter cell in successive drivecycles, wherein in the on-mode the at least one electronic switch is inan on-state throughout each drive cycle, wherein in the off-mode the atleast one electronic switch is in an off-state throughout each drivecycle, and wherein in the PWM mode, the at least one electronic switchis in the on-state for an on-period and in the off-state for anoff-period in each drive cycle.

B13. The power converter circuit of item B1, wherein the firstoperational characteristic includes a first voltage blocking capabilityof at least one electronic switch in the first converter cell, and thesecond operational characteristic includes a second voltage blockingcapability of at least one electronic switch in the second convertercell.

B14. The power converter circuit of item B13, wherein the first voltageblocking capability is less than 80% of the second voltage blockingcapability.

B15. The power converter circuit of item B13, wherein the voltageblocking capability of the at least one electronic switch in each of theplurality of converter cells is different from the voltage blockingcapability of the at least one electronic switch in each of the othersof the plurality of converter cells.

B16. The power converter circuit of item B13, wherein each of the firstconverter cell and the second converter cell includes a half-bridge, andwherein the at least one electronic switch of the first converter cellis a high-side switch of the respective half-bridge, and the at leastone electronic switch of the second converter cell is a high-side switchof the respective half-bridge.

B17. The power converter circuit of item B11, wherein each of the firstconverter cell and the second converter cell includes a half-bridge, andwherein the at least one electronic switch of the first converter cellis a low-side switch of the respective half-bridge, and the at least oneelectronic switch of the second converter cell is a low-side switch ofthe respective half-bridge.

B18. The power converter circuit of item B13, wherein each of theplurality of converter cells includes a cell input and a cell output,wherein the cell inputs of the plurality of converter cells areconnected in series.

B19. The power converter circuit of item B13, wherein each of theplurality of converter cells includes a cell input and a cell output,wherein the cell outputs of the plurality of converter cells areconnected in series.

B20. The power converter circuit of item B13, further including: aplurality of capacitors, wherein each of the plurality of capacitors isconnected to one of the plurality of converter cells; and a furtherpower converter coupled to the plurality of converter cells, wherein thefurther power converter includes at least one converter cell.

B21. The power converter circuit of item B1, wherein the firstoperational characteristic includes a first on-resistance of at leastone electronic switch in the first converter cell, and the secondoperational characteristic includes a second on-resistance of at leastone electronic switch in the second converter cell.

B22. The power converter circuit of item B21, wherein the firston-resistance is less than 80% of the second on-resistance.

B23. The power converter circuit of item B22, wherein the on-resistanceof the at least one electronic switch in each of the plurality ofconverter cells is different from the on-resistance of the at least oneelectronic switch in each of the others of the plurality of convertercells.

B24. The power converter circuit of item B22, wherein each of the firstconverter cell and the second converter cell includes a half-bridge, andwherein the at least one electronic switch of the first converter cellis a high-side switch of the respective half-bridge, and the at leastone electronic switch of the second converter cell is a high-side switchof the respective half-bridge.

B25. The power converter circuit of item B23, wherein each of the firstconverter cell and the second converter cell includes a half-bridge, andwherein the at least one electronic switch of the first converter cellis a low-side switch of the respective half-bridge, and the at least oneelectronic switch of the second converter cell is a low-side switch ofthe respective half-bridge.

B26. The power converter circuit of item B23, wherein each of theplurality of converter cells includes a cell input and a cell output,wherein the cell inputs of the plurality of converter cells areconnected in parallel.

B27. The power converter circuit of item B23, wherein each of theplurality of converter cells includes a cell input and a cell output,wherein the cell outputs of the plurality of converter cells areconnected in parallel.

B28. A method, including: receiving a periodic input voltage by a powerconverter including a plurality of converter cells each including a cellinput, and a cell output; and based on a voltage level of the periodicinput voltage, connecting the cell inputs of at least two convertercells of the plurality of converter cells either in parallel or inseries.

B29. The method of item B28, including: connecting the cell inputs inparallel when the instantaneous voltage level is below a predefinedvoltage threshold.

B30. The method of item B28, further including: receiving a cell outputpower at the cell output of each converter cell by another powerconverter.

B31. The method of item B30, wherein the multi-cell power converter andthe other power converter are linked by a plurality of capacitors.

B32. A method, including: receiving a periodic voltage by a powerconverter including a plurality of converter cells each including a celloutput, and a cell input; and based on an instantaneous voltage level ofthe output voltage, connecting the cell outputs of at least twoconverter cells of the plurality of converter cells either in parallelor in series.

B33. The method of item B32, including: connecting the cell outputs inparallel when the voltage level is below a predefined voltage threshold.

B34. The method of item B32, further including receiving a cell inputpower at the cell input of each converter cell from another powerconverter.

B35. The method of item 33, wherein the other converter and the furtherpower converter are linked by a plurality of capacitors.

C1. A method, including: receiving a periodic voltage by a powerconverter including a plurality of converter cells; in a series of timeframes of equal duration, alternating an average power level of powerconverted by at least one converter cell of the plurality of convertercells, wherein each of the series of time frames corresponds to a timeperiod between sequential zero crossings of the periodic voltage.

C2. The method of item C1, wherein the periodic voltage is one of a sinevoltage, and a rectified sine voltage.

C3. The method of item C1, wherein the series of time frames includes afirst number (P) of times frames, and a second number (Q) of timeframes, and wherein alternating the average power level includesconverting power for each of the first number of time frames andconverting power for each of the second plurality of time frames suchthat an average power level in each of the second number of time framesis lower than an average power level in each of the first number of timeframes.

C4. The method of item C3, wherein alternating the average power levelin the series of time frames includes sequentially alternating theaverage power level.

C5. The method of item C3, wherein the average power level in the secondnumber of time frames is less than 50% of the average power level in thefirst number of time frames.

C6. The method of one of items C3-05, wherein the average power level inthe second number of time frames is zero.

C7. The method of one of items C3-C6, wherein a ratio between the secondnumber of time frames and the first number of time frames is higher than1.

C8. The method of one of items C3-C6, wherein alternating an averagepower level of power converted by at least one converter cell of theplurality of converter cells includes alternating an average power levelof power converted by each of the plurality of converter cells.

C9. The method of one of items C3-C8, wherein the power converterincludes an input and wherein the periodic voltage is received at theinput.

010. The method of item C9, wherein each of the plurality of convertercells includes a cell input and a cell output, and wherein the cellinputs of the converter cells are connected in series, wherein a seriescircuit which includes the cell inputs is coupled to the input of thepower converter.

011. The method of one of items C9-010, wherein each of the plurality ofconverter cells includes a cell input and a cell output, and wherein thecell inputs of the converter cells are connected in parallel, wherein aparallel circuit which includes the cell inputs is coupled to the inputof the power converter.

C12. The method of one of items 010-011, further including: receivingpower from the power converter by another power converter, wherein theother power converter is coupled to the cell outputs of the plurality ofconverter cells.

C13. The method of item C12, each of the cell outputs of the pluralityof converter cells is coupled to a respective capacitor of a pluralityof capacitors.

C14. The method of item C1, wherein the power converter includes anoutput and wherein the periodic voltage is received at the output.

C15. The method of item C14, wherein each of the converter cellsincludes a cell input and a cell output, and wherein the cell outputs ofthe converter cells are connected in series, and wherein a seriescircuit which includes the cell outputs is coupled to the output of thepower converter.

C16. The method of item C14, wherein each of the converter cellsincludes a cell input and a cell output, and wherein the cell outputs ofthe converter cells are connected in parallel, wherein a parallelcircuit which includes the cell outputs is coupled to the output of thepower converter.

C17. The method of one of items C14-C16, further including: receivingpower by the power converter from another further power converter,wherein the other power converter is coupled to the cell inputs of theplurality of converter cells.

C18. The method of item C17, wherein each cell input of the plurality ofconverter cells is coupled to a respective one of a plurality ofcapacitors.

C19. A method, including: converting DC power by a power converterincluding a plurality of converter cells in a first mode or a secondmode, wherein in the first mode a power level of the converted power issubstantially constant, and wherein in the second mode the power levelof the converted power alternates.

C20. The method of item C19, wherein converting the DC power in thesecond mode includes alternatingly operating at least one of theplurality of converter cells in one of an active mode and an inactivemode.

C21. The method of item C20, wherein operating the at least one of theplurality of converter cells in the inactive mode includes convertingzero power by the at least one of the plurality of converter cells.

C22. The method of one of items C20-C21, wherein converting the DC powerin the second mode includes operating only one converter cell in theactive mode at one time.

C23. The method of item C22, wherein converting the DC power in thesecond mode includes operating each of the plurality of converter cellsat different times.

C24. The method of one of items C19-C23, wherein converting DC powerincludes converting DC power in the first mode or the second mode basedon a level of a power reference signal.

C25. The method of item C24, wherein in the first mode the power levelof the converted power is dependent on a level of the power referencesignal.

C26. The method of item C24, wherein converting the DC power includesconverting the DC power in the second mode when the level of the powerreference signal falls below a predefined threshold.

C27. The method of one of items C24-C26, wherein converting the DC powerin the second mode includes converting the DC power such that an averagepower level in the second mode is dependent on the level of the powerreference signal.

C28. The method of one of item C19-C27, wherein an average power levelin the second mode is less than 50% of the power level in the firstmode.

C29. The method of one of items C24-C28, wherein converting the DC powerincludes providing an output current at an output of the powerconverter, and wherein the power reference signal includes an outputcurrent reference signal.

C30. The method of item C29, further including: receiving power by thepower converter from another power converter.

C31. The method of item C19, wherein each of the plurality of convertercells includes a cell input and a cell output, and wherein cell outputsof the plurality of converter cells are connected in parallel, wherein aparallel circuit which includes the cell outputs is coupled to theoutput of the power converter.

C32. The method of item C19, wherein each of the plurality of convertercells includes a cell input and a cell output, and wherein cell outputsof the plurality of converter cells are connected in series, wherein aseries circuit which includes the cell outputs is coupled to the outputof the power converter.

C33. The method of item C19, wherein converting the DC power includesreceiving an input current at an input of the power converter, andwherein the power reference signal includes an input current referencesignal.

C34. The method of item C33, further including: receiving power from thepower converter by another power converter.

C35. The method of item C19, wherein each of the plurality of convertercells includes a cell input and a cell output, and wherein the cellinputs of the plurality of converter cells are connected in parallel,wherein a parallel circuit which includes the cell inputs is coupled tothe output of the power converter.

C36. The method of item C19, wherein each of the plurality of convertercells includes a cell input and a cell output, and wherein the cellinputs of the plurality of converter cells are connected in series,wherein a series circuit which includes the cell inputs is coupled tothe output of the power converter.

C37. A power converter circuit including: a power converter including aplurality of converter cells, wherein the power converter is configuredto receive a periodic voltage; and a controller configured in a seriesof time frames to alternate an average power level of power converted byat least one converter cell of the plurality of converter cells, whereineach time frame corresponds to a time period between sequential zerocrossings of the periodic signal.

C38. The power converter circuit of item C37, wherein the periodicvoltage is one of a sine voltage, and a rectified sine voltage.

C39. The power converter circuit of item C37, wherein the powerconverter includes an input, wherein the power converter is configuredto receive the periodic voltage at the input, and wherein the powerconverter circuit is configured to output power to another powerconverter.

C40. The power converter circuit of item C37, wherein the powerconverter includes an output, wherein the power converter is configuredto receive the periodic voltage at the output, and wherein the powerconverter circuit includes another power converter configured to supplypower to the power converter.

C41. A power converter circuit including: a power converter including aplurality of converter cells and a controller, wherein the controller isconfigured to operate the power converter in one of a first mode and asecond mode, wherein in the first mode a power level of the convertedpower is substantially constant, and wherein in the second mode thepower level of the converted power alternates.

C42. The power converter circuit of item C41, wherein the controller isconfigured to operate at least one of the plurality of converter cellsalternatingly in one of an active mode and an inactive mode.

C43. The power converter circuit of item C42, wherein the controller isconfigured to operate the at least one of the plurality of convertercells in the inactive mode such that the at least one of the pluralityof converter cells converts zero power.

C44. The power converter circuit of item C41, wherein the controller isconfigured to operate only one of the plurality of converter cells inthe active mode at one time.

D1. A method, including: converting power by a power converter includinga plurality of converter cells and at least one filter cell; receiving acell input power at a cell input and providing a cell output power at acell output of at least one of the plurality of converter cells; andoperating the filter cell in one of an input power mode, in which thefilter cell receives an input power, and an output power mode in, inwhich the filter cell provides an output power.

D2. The method of item D1, wherein operating the filter cell in theinput power mode includes receiving the input power at a terminal of thefilter cell, and wherein operating the filter cell in the output powermode includes providing the output power at the terminal of the filtercell.

D3. The method of item D1, wherein each of the plurality converter cellsincludes a cell input and a cell output, and wherein the cell inputs ofthe plurality of converter cells and the terminal of the at least onefilter cell are connected in series, wherein a series circuit whichincludes the cell inputs and terminal is coupled to an input of thepower converter.

D4. The method of item D3, further including: receiving an input voltageat the input of the power converter; providing a cell output voltage byeach of the plurality of converter cells; and operating the filter cellin the one of the input power mode and the output power mode based on avoltage level of the input voltage and on voltage levels of the celloutput voltages.

D5. The method of item D4, wherein operating the filter cell in the oneof the input power mode and the output power mode includes: operating afirst number of the plurality of converter cells in an off-state, andoperating a second number of the converter cells in an on-state,operating the filter cell in the input power mode, when a sum of thecell output voltages of the first number of converter cells is lowerthan the level of the input voltage, and operating the filter cell inthe output power mode, when a sum of the cell output voltages of thefirst number of converter cells is higher than the level of the inputvoltage.

D6. The method of item D5, wherein the first number is dependent on thelevel of the input voltage.

D7. The method of item D4, wherein the input voltage is a periodicvoltage.

D8. The method of item D3, further including: receiving an input voltageand an input current at the input of the power converter, andcontrolling the input current to have a predefined phase differencerelative to the input voltage.

D9. The method of item D8, further including: controlling a cell outputvoltage at the cell output of each of the plurality of converter cells.

D10. The method of one of items D2-D9, further including: receiving acell output power provided by each of the plurality of converter cellsby another power converter.

D11. The method of item D10, wherein the other power converter includesonly one converter cell.

D12. The method of item D10, wherein the other power converter includesa plurality of converter cells, wherein each of the plurality ofconverter cells of the other power converter receives the cell outputpower provided by an associated converter cell of the plurality ofconverter cells of the power converter.

D13. The method of item D10, wherein the power converter and the otherpower converter are linked by a plurality of capacitors.

D14. The method of item D1, wherein each of the plurality convertercells includes a cell input and a cell output, and wherein the celloutputs of the plurality of converter cells and the terminal of thefilter cell are connected in series, wherein a series circuit whichincludes the cell outputs and the terminal is coupled to an output ofthe power converter.

D15. The method of item D14, further including: receiving an outputvoltage at the output of the power converter; receiving a cell inputvoltage by each converter cell; and operating the filter cell in the oneof the input power mode and the output power mode based on a voltagelevel of the output voltage and on voltage levels of the cell inputvoltages.

D16. The method of item D15, wherein operating the filter cell in theone of the input power mode and the output power mode includes:operating a first number of the plurality of converter cells in anoff-state, and operating a second number of the converter cells in anon-state, operating the filter cell in the output power mode, when a sumof the cell input voltages of the first number of converter cells islower than the level of the output voltage, and operating the filtercell in the input power mode, when a sum of the cell input voltages ofthe first number of converter cells is higher than the level of theoutput voltage.

D17. The method of item D16, wherein the first number is dependent onthe level of the output voltage.

D18. The method of item D16, wherein the output voltage is a periodicvoltage.

D19. The method of one of items D14, further including:

receiving an output voltage and providing an output current at theoutput of the power converter, and controlling the output current tohave a predefined phase difference relative to the output voltage.

D20. The method of one of items D14-D19, further including: providing acell input power received by each of the plurality of converter cells byanother power converter.

D21. The method of item D20, wherein the other power converter includesonly one converter cell.

D22. The method of item D20, wherein the other power converter includesa plurality of converter cells; and wherein each of the plurality ofconverter cells of the power converter receives the cell input powerfrom an associated converter cell of the plurality of converter cells ofthe other power converter.

D23. The method of one of items D20-D22 wherein the power converter andthe further power converter are linked by a plurality of capacitors.

D24. A power converter circuit including power converter, whichincludes: a plurality of converter cells, wherein at least one of theplurality of converter cells is configured to receive a cell input powerat a cell input and provide a cell output power at a cell output; and atleast one filter cell configured to operate in one of an input powermode, in which the filter cell receives an input power, and an outputpower mode in, in which the filter cell provides an output power.

D25. The power converter circuit of item D24, wherein the filter cell isconfigured to receive the input power at a terminal of the filter cellin the input power mode, and to provide the output power at the terminalof the filter cell in the output power mode.

D26. The power converter circuit of item D24, wherein each of theplurality converter cells includes a cell input and a cell output, andwherein the cell inputs of the plurality of converter cells and theterminal of the filter cell are connected in series, wherein a seriescircuit which includes the cell inputs and the terminal is coupled to aninput of the power converter.

D27. The power converter circuit of item D26, wherein the powerconverter is configured to receive an input voltage at the input of thepower converter; wherein each converter cell is configured to provide acell output voltage; and wherein the filter cell is configured tooperate in the one of the input power mode and the output power modebased on a voltage level of the input voltage and on voltage levels ofthe cell output voltages.

D28. The power converter circuit of item D27, wherein a first number ofthe plurality of converter cells is configured to operate in anoff-state, and a second number of the plurality of converter cells isconfigured to operate in an on-state, wherein the filter cell isconfigured to operate in the input power mode, when a sum of the celloutput voltages of the first number of converter cells is lower than thelevel of the input voltage, and to operate in the output power mode,when a sum of the cell output voltages of the first number of convertercells is higher than the level of the input voltage.

D29. The power converter circuit of item D28, wherein the first numberis dependent on the level of the input voltage.

D30. The method of one of items D16-D29, wherein the input voltage is aperiodic voltage.

D31. The power converter circuit of item D26, wherein the powerconverter is configured to receive an input voltage and an input currentat the input of the power converter, and to control the input current tohave a predefined phase difference relative to the input voltage.

D32. The power converter circuit of item 31, wherein the power converteris further configured to control a cell output voltage at the celloutput of each of the plurality of converter cells.

D33. The power converter circuit of one of items D20-D32, furtherincluding: another power converter configured to receive a cell outputpower provided by each of the plurality of converter cells.

D34. The power converter circuit of item D33, wherein the other powerconverter includes only one converter cell.

D35. The power converter circuit of item D33, wherein the further powerconverter includes a plurality of converter cells, and wherein each ofthe plurality of converter cells of the other power converter receivesthe cell output power provided by one of the plurality of convertercells of the power converter.

D36. The power converter circuit of one of items D33-D35, wherein thepower converter and the other power converter are linked by a pluralityof capacitors.

D37. The power converter circuit of one of items D24-D36, wherein eachof the plurality converter cells includes a cell input and a celloutput, and wherein the cell outputs of the plurality of converter cellsand the terminal of the filter cell are connected in series, wherein aseries circuit which includes the cell outputs and the terminal iscoupled to an output of the power converter.

D38. The power converter circuit of item D37, wherein the powerconverter is configured to receive an output voltage at the output ofthe power converter; wherein each converter cell is configured toreceive a cell input voltage; and wherein the filter cell is configuredto operate in the one of the input power mode and the output power modebased on a voltage level of the output voltage and on voltage levels ofthe cell input voltages.

D39. The power converter circuit of item D38, wherein a first number ofthe plurality of converter cells is configured to operate in anoff-state, and a second number of the plurality of converter cells isconfigured to operate in an on-state, wherein the filter cell isconfigured to operate in the input power mode, when a sum of the cellinput voltages of the first number of converter cells is lower than thelevel of the output voltage, and to operate in the output power mode,when a sum of the cell input voltages of the first number of convertercells is higher than the level of the output voltage.

D40. The power converter circuit of item D39, wherein the first numberis dependent on the level of the output voltage.

D41. The method of one of items D38-D40, wherein the output voltage is aperiodic voltage.

D42. The power converter circuit of one of items D37-D41, wherein thepower converter is configured to receive an output voltage and toprovide an output current at the output of the power converter, and tocontrol the output current to have a predefined phase differencerelative to the output voltage.

D43. The power converter circuit of item D42, wherein the powerconverter is further configured to control a cell input voltage at thecell input of each of the plurality of converter cells.

D44. The power converter circuit of one of items D37-D43, furtherincluding: another power converter configured to provide a cell inputpower to each of the plurality of converter cells.

D45. The power converter circuit of item D44, wherein the other powerconverter includes at least one converter cell.

D46. The power converter circuit of one of items D44-D45, wherein theother power converter includes a plurality of converter cells, andwherein each of the plurality of converter cells of the other powerconverter provides the cell input to one of the plurality of convertercells of the power converter.

D47. The power converter circuit of one of items D44-D46, wherein thepower converter and the other power converter are linked by a pluralityof capacitors.

E1. A power converter circuit, including a power converter with aplurality of series connected converter cells, wherein each of theplurality of converter cells includes at least one first half-bridgecircuit including a first silicon MOSFET (Metal Oxide SemiconductorField-Effect Transistor) and a second silicon MOSFET, and wherein atleast one of the plurality of converter cells is configured to operatein a continuous current mode.

E2. The power converter circuit of item E1, wherein in the continuouscurrent mode, a current in the first half-bridge is different from zero.

E3. The power converter circuit of one of items E1-E2, wherein each ofthe plurality of converter cells is configured to operate in acontinuous current mode.

E4. The power converter circuit of one of items E1-E3, wherein each ofthe plurality of converter cells further includes a second half-bridgeincluding a third silicon MOSFET and a fourth silicon MOSFET.

E5. The power converter circuit of item E4, wherein at least one of theplurality of converter cells is configured to receive a periodic voltageand to operate in a totem pole modulation mode.

E6. The power converter circuit of item E5, wherein the at least oneconverter cell in the totem pole modulation mode is configured tooperate one of the first and second half-bridges at a first frequencywhich is dependent on a frequency of the periodic voltage, and tooperate the other one of the first and second half-bridges at a secondfrequency higher than the frequency of the periodic voltage.

E7. The power converter circuit of item E6, wherein the first frequencyis twice the frequency of the periodic voltage.

E8. The power converter circuit of item E6, wherein the second frequencyis at least 200 times the frequency of the periodic voltage.

E9. The power converter circuit of one of items E1-E8, wherein each ofthe plurality of converter cells includes a cell input and a celloutput, and wherein the cell inputs of the plurality of converter cellsare connected in series and wherein a series circuit which includes thecell inputs is coupled to an input of the power converter.

E10. The power converter circuit of item E9, wherein the power converterfurther includes at least one inductor connected in series with the cellinputs.

E11. The power converter circuit of item E9, further including aplurality of capacitors, wherein each of the cell outputs connected to arespective one of the plurality of capacitors.

E12. The power converter circuit of item E1, wherein each of theplurality of converter cells includes a cell input and a cell output,and wherein the cell outputs of the plurality of converter cells areconnected in series, wherein a series circuit which includes the celloutputs of the plurality of converter cells is coupled to an output ofthe power converter.

E13. The power converter circuit of item E12, wherein the powerconverter further includes at least one inductor connected in serieswith the cell outputs.

E14. The power converter circuit of item E12, further including aplurality of capacitors, wherein the cell input of each of the pluralityof converter cells is connected to a respective one of the plurality ofcapacitors.

E15. The power converter circuit of one of items E1-E14, furtherincluding: another power converter coupled to the power converter.

E16. The power converter circuit of one of items E1-E15, wherein each ofthe first and second silicon MOSFETs has a voltage blocking capabilitygreater than 100V.

E17. The power converter circuit of one of items E1-E16, wherein thefirst silicon MOSFET includes a first on-resistance and a first voltageblocking capability, and the second silicon MOSFET includes a secondon-resistance and a second voltage blocking capability, wherein thefirst voltage blocking capability and the second voltage blockingcapability are substantially equal, and wherein the first on-resistanceis different from the second on-resistance.

E18. The power converter circuit of item E17, wherein the firston-resistance is less than 90% of the second on-resistance.

E19. A method including: operating at least one converter cell of apower converter including a plurality of series connected convertercells in a continuous current mode, wherein each of the converter cellsincludes at least a first half-bridge including a first silicon MOSFETand a second silicon MOSFET.

E20. The method of item E19, wherein operating the at least oneconverter cell in the continuous current mode includes operating thefirst half-bridge such that, a current in the first half-bridge isdifferent from zero.

E21. The method of item E20, wherein operating at least one convertercell in the continuous current mode includes operating each of theplurality of converter cells in the continuous current mode.

E22. The method of one of items E19-E21, wherein each of the pluralityof converter cells further includes a second half-bridge including athird silicon MOSFET and a fourth silicon MOSFET.

E23. The method of item E22, wherein operating the at least oneconverter cell in the continuous current mode includes: receiving aperiodic voltage by the least one converter cells, and operating the atleast one converter cell in a totem pole modulation mode.

E24. The method of item E23, wherein operating the at least oneconverter cell in the totem pole modulation mode includes operating oneof the first and second half-bridges at a first frequency which isdependent on a frequency of the periodic voltage, and operating theother one of the first and second half-bridges at a second frequencyhigher than the frequency of the periodic voltage.

E25. The method of item E24, wherein the first frequency is twice thefrequency of the periodic voltage.

E26. The method of item E25, wherein the second frequency is at least200 times the frequency of the periodic voltage.

E27. The method of one of items E19-E26, wherein a voltage blockingcapability of each of the first and second silicon MOSFETs is higherthan 100V.

E28. A power converter circuit, including: a power converter with aplurality of converter cells, wherein each converter cell includes at ahalf-bridge circuit including a first electronic switch and a secondelectronic switch, wherein the first electronic switch includes a firston-resistance and a first voltage blocking capability, and the secondelectronic switch includes a second on-resistance and a second voltageblocking capability, wherein in at least one converter cell the firstvoltage blocking capability and the second voltage blocking capabilityare substantially equal, and the first on-resistance and the secondon-resistance are different.

E29 The power converter circuit of item E28, wherein in the at least oneconverter cell the first on-resistance is less than 90% of the secondon-resistance.

E30. The power converter circuit of item E29, wherein the firston-resistance is less than 80% of the second on-resistance.

E31. The power converter circuit of item E28, wherein in the at leastone converter cell, the first voltage blocking capability is between 90%and 110% of the second voltage blocking capability.

E32. The power converter circuit of item E28, wherein each of theplurality of converter cell includes a cell input and a cell output, andwherein the cell inputs of the plurality of converter cells areconnected in series, and wherein a series circuit which includes thecell inputs of the plurality of converter cells is coupled to an inputof the multi-cell power converter.

E33. The power converter circuit of item E32, further including aninductor connected in series with the cell inputs.

E34. The power converter circuit of item E32, further including aplurality of capacitors, wherein the cell output of each converter cellis connected to a respective one of the plurality of capacitors.

E35. The power converter circuit of item E28, wherein each of theplurality of converter cell includes a cell input and a cell output, andwherein the cell outputs of the plurality of converter cells areconnected in series, wherein a series circuit which includes the celloutputs of the plurality of converter cells is coupled to an output ofthe power converter.

E36. The power converter circuit of item E35, further including aninductor connected in series with the cell outputs.

E37. The power converter circuit of item E35, further including aplurality of capacitors, wherein the cell input of each converter cellis connected to a respective one of the plurality capacitors.

E38. The power converter circuit of item E28, wherein each of theplurality of converter cells includes a cell input and a cell output,and wherein the cell outputs of the plurality of converter cells areconnected in parallel, wherein a parallel circuit which includes thecell outputs of the plurality of converter cells is coupled to an outputof the power converter.

E39. The power converter circuit of item E28, wherein each of theplurality of converter cells includes a cell input and a cell output,and wherein the cell inputs of the plurality of converter cells areconnected in parallel, wherein a parallel circuit which includes thecell inputs of the plurality of converter cells is coupled to an inputof the power converter.

E40. The power converter circuit of item E28, wherein each of the firstand second electronic switches is selected from the group consisting of:a MOSFET (Metal Oxide Semiconductor Field Effect Transistor); a HEMT(High Electron Mobility Transistor); a JFET (Junction Field EffectTransistor); an IGBT (Insulated Gate Bipolar Transistor); and a BJT(Bipolar Junction Transistor).

E41. The power converter circuit of one of items E28-E40, furtherincluding: a plurality of capacitors wherein each of the plurality ofcapacitors is coupled to a respective one of the plurality of convertercells; and another power converter coupled to the plurality ofcapacitors.

F1. A power converter circuit including a power converter which includesa plurality of converter cells configured to operate in a PWM mode,wherein the plurality of converter cells includes a first converter cellconfigured to operate in the PWM mode at a first switching frequency,and a second converter cell configured to operate in the PWM mode at asecond switching frequency different than the first frequency.

F2. The power converter circuit of item F1, wherein the second switchingfrequency is at least 1.2 times the first switching frequency.

F3. The power converter circuit of one of items F1-F2, wherein the powerconverter includes a topology selected from a group consisting of: IS,OS, IP, OP.

F4. The power converter circuit of one of items F1-F3, wherein the powerconverter includes one of an IS and OS topology, wherein the powerconverter is configured to receive a periodic voltage, and wherein thefirst converter cell and the second converter cell are configured tooperate at substantially the same modulation index and duty cycle,respectively.

G1. A method, including: operating a first converter cell in amulti-cell in a PWM mode at a first frequency, and operating a secondconverter cell in the multi-cell in a PWM mode at a second frequencydifferent than the first frequency.

H1. A method, including: receiving input power by a multi-cell converterand supplying output power to a plurality of separate loads, wherein themulti-cell power converter includes an IS topology.

H2. The method of item H1, wherein the multi-cell converter includes aplurality of converter cells, and wherein each converter cell suppliespower to a respective load.

H3. The method of one of items H1-H2, wherein the input power isreceived from an AC power grid.

I1. A method including: receiving input power by a multi-cell converterfrom a plurality of separate power sources and supplying output power toa load.

I2. The method of item I1, wherein the load is an AC power grid.

I3. The method of one of items I1-I2, wherein the multi-cell converterincludes an OS topology.

Although various exemplary embodiments of the invention have beendisclosed, it will be apparent to those skilled in the art that variouschanges and modifications can be made which will achieve some of theadvantages of the invention without departing from the spirit and scopeof the invention. It will be obvious to those reasonably skilled in theart that other components performing the same functions may be suitablysubstituted. It should be mentioned that features explained withreference to a specific figure may be combined with features of otherfigures, even in those cases in which this has not explicitly beenmentioned. Further, the methods of the invention may be achieved ineither all software implementations, using the appropriate processorinstructions, or in hybrid implementations that utilize a combination ofhardware logic and software logic to achieve the same results. Suchmodifications to the inventive concept are intended to be covered by theappended items.

Spatially relative terms such as “under,” “below,” “lower,” “over,”“upper” and the like, are used for ease of description to explain thepositioning of one element relative to a second element. These terms areintended to encompass different orientations of the device in additionto different orientations than those depicted in the figures. Further,terms such as “first,” “second” and the like, are also used to describevarious elements, regions, sections, etc. and are also not intended tobe limiting. Like terms refer to like elements throughout thedescription.

As used herein, the terms “having,” “containing,” “including,”“including” and the like are open ended terms that indicate the presenceof stated elements or features, but do not preclude additional elementsor features. The articles “a,” “an” and “the” are intended to includethe plural as well as the singular, unless the context clearly indicatesotherwise.

With the above range of variations and applications in mind, it shouldbe understood that the present invention is not limited by the foregoingdescription, nor is it limited by the accompanying drawings. Instead,the present invention is limited only by the following items and theirlegal equivalents.

1. A method, comprising: converting power by a power convertercomprising a plurality of converter cells and at least one filter cell;receiving a cell input power at a cell input and providing a cell outputpower at a cell output of at least one of the plurality of convertercells; and operating the filter cell in one of an input power mode, inwhich the filter cell receives an input power, and an output power mode,in which the filter cell provides an output power.
 2. The method ofclaim 1, wherein operating the filter cell in the input power modecomprises receiving the input power at a terminal of the filter cell,and wherein operating the filter cell in the output power mode comprisesproviding the output power at the terminal of the filter cell.
 3. Themethod of claim 1, wherein each of the plurality converter cellscomprises a cell input and a cell output, and wherein the cell inputs ofthe plurality of converter cells and the terminal of the at least onefilter cell are connected in series, wherein a series circuit whichincludes the cell inputs and terminal is coupled to an input of thepower converter.
 4. The method of claim 3, further comprising: receivingan input voltage at the input of the power converter; providing a celloutput voltage by each of the plurality of converter cells; andoperating the filter cell in the one of the input power mode and theoutput power mode based on a voltage level of the input voltage and onvoltage levels of the cell output voltages.
 5. The method of claim 4,wherein operating the filter cell in the one of the input power mode andthe output power mode comprises: operating a first number of theplurality of converter cells in an off-state, and operating a secondnumber of the converter cells in an on-state, operating the filter cellin the input power mode, when a sum of the cell output voltages of thefirst number of converter cells is lower than the level of the inputvoltage, and operating the filter cell in the output power mode, when asum of the cell output voltages of the first number of converter cellsis higher than the level of the input voltage.
 6. The method of claim 5,wherein the first number is dependent on the level of the input voltage.7. The method of claim 4, wherein the input voltage is a periodicvoltage.
 8. The method of claim 3, further comprising: receiving aninput voltage and an input current at the input of the power converter,and controlling the input current to have a predefined phase differencerelative to the input voltage.
 9. The method of claim 8, furthercomprising: controlling a cell output voltage at the cell output of eachof the plurality of converter cells.
 10. The method of claim 2, furthercomprising: receiving a cell output power provided by each of theplurality of converter cells by another power converter.
 11. The methodof claim 10, wherein the other power converter comprises only oneconverter cell.
 12. The method of claim 10, wherein the other powerconverter comprises a plurality of converter cells, wherein each of theplurality of converter cells of the other power converter receives thecell output power provided by an associated converter cell of theplurality of converter cells of the power converter.
 13. The method ofclaim 10, wherein the power converter and the other power converter arelinked by a plurality of capacitors.
 14. The method of claim 1, whereineach of the plurality converter cells comprises a cell input and a celloutput, and wherein the cell outputs of the plurality of converter cellsand the terminal of the filter cell are connected in series, wherein aseries circuit which includes the cell outputs and the terminal iscoupled to an output of the power converter.
 15. The method of claim 14,further comprising: receiving an output voltage at the output of thepower converter; receiving a cell input voltage by each converter cell;and operating the filter cell in the one of the input power mode and theoutput power mode based on a voltage level of the output voltage and onvoltage levels of the cell input voltages.
 16. The method of claim 15,wherein operating the filter cell in the one of the input power mode andthe output power mode comprises: operating a first number of theplurality of converter cells in an off-state, and operating a secondnumber of the converter cells in an on-state, operating the filter cellin the output power mode, when a sum of the cell input voltages of thefirst number of converter cells is lower than the level of the outputvoltage, and operating the filter cell in the input power mode, when asum of the cell input voltages of the first number of converter cells ishigher than the level of the output voltage.
 17. The method of claim 16,wherein the first number is dependent on the level of the outputvoltage.
 18. The method of claim 16, wherein the output voltage is aperiodic voltage.
 19. The method of claim 14, further comprising:receiving an output voltage and providing an output current at theoutput of the power converter, and controlling the output current tohave a predefined phase difference relative to the output voltage. 20.The method of claim 14, further comprising: providing a cell input powerreceived by each of the plurality of converter cells by another powerconverter.
 21. The method of claim 20, wherein the other power convertercomprises only one converter cell.
 22. The method of claim 20, whereinthe other power converter comprises a plurality of converter cells; andwherein each of the plurality of converter cells of the power converterreceives the cell input power from an associated converter cell of theplurality of converter cells of the other power converter.
 23. Themethod of claim 20 wherein the power converter and the further powerconverter are linked by a plurality of capacitors.
 24. A power convertercircuit comprising power converter, which comprises: a plurality ofconverter cells, wherein at least one of the plurality of convertercells is configured to receive a cell input power at a cell input andprovide a cell output power at a cell output; and at least one filtercell configured to operate in one of an input power mode, in which thefilter cell receives an input power, and an output power mode in, inwhich the filter cell provides an output power.
 25. The power convertercircuit of claim 24, wherein the filter cell is configured to receivethe input power at a terminal of the filter cell in the input powermode, and to provide the output power at the terminal of the filter cellin the output power mode.
 26. The power converter circuit of claim 24,wherein each of the plurality converter cells comprises a cell input anda cell output, and wherein the cell inputs of the plurality of convertercells and the terminal of the filter cell are connected in series,wherein a series circuit which includes the cell inputs and the terminalis coupled to an input of the power converter.
 27. The power convertercircuit of claim 26, wherein the power converter is configured toreceive an input voltage at the input of the power converter; whereineach converter cell is configured to provide a cell output voltage; andwherein the filter cell is configured to operate in the one of the inputpower mode and the output power mode based on a voltage level of theinput voltage and on voltage levels of the cell output voltages.
 28. Thepower converter circuit of claim 27, wherein a first number of theplurality of converter cells is configured to operate in an off-state,and a second number of the plurality of converter cells is configured tooperate in an on-state, and wherein the filter cell is configured tooperate in the input power mode, when a sum of the cell output voltagesof the first number of converter cells is lower than the level of theinput voltage, and to operate in the output power mode, when a sum ofthe cell output voltages of the first number of converter cells ishigher than the level of the input voltage.
 29. The power convertercircuit of claim 28, wherein the first number is dependent on the levelof the input voltage.
 30. The method of claim 16, wherein the inputvoltage is a periodic voltage.
 31. The power converter circuit of claim26, wherein the power converter is configured to receive an inputvoltage and an input current at the input of the power converter, and tocontrol the input current to have a predefined phase difference relativeto the input voltage.
 32. The power converter circuit of claim 31,wherein the power converter is further configured to control a celloutput voltage at the cell output of each of the plurality of convertercells.
 33. The power converter circuit of claim 20, further comprising:another power converter configured to receive a cell output powerprovided by each of the plurality of converter cells.
 34. The powerconverter circuit of claim 33, wherein the other power convertercomprises only one converter cell.
 35. The power converter circuit ofclaim 33, wherein the further power converter comprises a plurality ofconverter cells, and wherein each of the plurality of converter cells ofthe other power converter receives the cell output power provided by oneof the plurality of converter cells of the power converter.
 36. Thepower converter circuit of claim 33, wherein the power converter and theother power converter are linked by a plurality of capacitors.
 37. Thepower converter circuit of claim 24, wherein each of the pluralityconverter cells comprises a cell input and a cell output, and whereinthe cell outputs of the plurality of converter cells and the terminal ofthe filter cell are connected in series, wherein a series circuit whichincludes the cell outputs and the terminal is coupled to an output ofthe power converter.
 38. The power converter circuit of claim 37,wherein the power converter is configured to receive an output voltageat the output of the power converter; wherein each converter cell isconfigured to receive a cell input voltage; and wherein the filter cellis configured to operate in the one of the input power mode and theoutput power mode based on a voltage level of the output voltage and onvoltage levels of the cell input voltages.
 39. The power convertercircuit of claim 38, wherein a first number of the plurality ofconverter cells is configured to operate in an off-state, and a secondnumber of the plurality of converter cells is configured to operate inan on-state, and wherein the filter cell is configured to operate in theinput power mode, when a sum of the cell input voltages of the firstnumber of converter cells is lower than the level of the output voltage,and to operate in the output power mode, when a sum of the cell inputvoltages of the first number of converter cells is higher than the levelof the output voltage.
 40. The power converter circuit of claim 39,wherein the first number is dependent on the level of the outputvoltage.
 41. The method of claim 38, wherein the output voltage is aperiodic voltage.
 42. The power converter circuit of claim 37, whereinthe power converter is configured to receive an output voltage and toprovide an output current at the output of the power converter, and tocontrol the output current to have a predefined phase differencerelative to the output voltage.
 43. The power converter circuit of claim42, wherein the power converter is further configured to control a cellinput voltage at the cell input of each of the plurality of convertercells.
 44. The power converter circuit of claim 37, further comprising:another power converter configured to provide a cell input power to eachof the plurality of converter cells.
 45. The power converter circuit ofclaim 44, wherein the other power converter comprises at least oneconverter cell.
 46. The power converter circuit of claim 44, wherein theother power converter comprises a plurality of converter cells, andwherein each of the plurality of converter cells of the other powerconverter provides the cell input to one of the plurality of convertercells of the power converter.
 47. The power converter circuit of claim44, wherein the power converter and the other power converter are linkedby a plurality of capacitors.